Python tree-sitter to CAST Porting: Imports (#826)

This PR introduces support for generating CAST import statements. This
is using tree sitter, as part of the ongoing effort to port over the
Python AST to CAST generation to use tree-sitter.

### Summary of Changes

- Adds handlers to python/ts2cast.py to support CAST generation for
import statements.
- Adds basic CAST generation for the attribute idiom. In order to
support function calls from imports.
- Adds test script test_import_cast.py with unit tests to maintain
consistency.
- Added 'support' for yields and assert statements. The current
generation is to passthrough them, so nothing important gets generated
from these.

### Related issues
-

Resolves #804

---------

Co-authored-by: Vincent Raymond <vincent@lum.ai> 102dd72

api/python/skema/img2mml/models/decoders/xfmer_decoder.html

@@ -365,7 +365,7 @@ <h3>Instance variables</h3>
 <h3>Methods</h3>
 <dl>
 <dt id="skema.img2mml.models.decoders.xfmer_decoder.Transformer_Decoder.create_pad_mask"><code class="name flex">
-<span>def <span class="ident">create_pad_mask</span></span>(<span>self, matrix: <built-in method tensor of type object at 0x7fe9c655b500>, pad_token: int) ‑> <built-in method tensor of type object at 0x7fe9c655b500></span>
+<span>def <span class="ident">create_pad_mask</span></span>(<span>self, matrix: <built-in method tensor of type object at 0x7f93e855b500>, pad_token: int) ‑> <built-in method tensor of type object at 0x7f93e855b500></span>
 </code></dt>
 <dd>
 <div class="desc"></div>

api/python/skema/img2mml/models/encoding/positional_encoding_for_xfmer.html

@@ -157,7 +157,7 @@ <h3>Class variables</h3>
 <h3>Methods</h3>
 <dl>
 <dt id="skema.img2mml.models.encoding.positional_encoding_for_xfmer.PositionalEncoding.forward"><code class="name flex">
-<span>def <span class="ident">forward</span></span>(<span>self, x: <built-in method tensor of type object at 0x7fe9c655b500>) ‑> <built-in method tensor of type object at 0x7fe9c655b500></span>
+<span>def <span class="ident">forward</span></span>(<span>self, x: <built-in method tensor of type object at 0x7f93e855b500>) ‑> <built-in method tensor of type object at 0x7f93e855b500></span>
 </code></dt>
 <dd>
 <div class="desc"><p>Defines the computation performed at every call.</p>

api/python/skema/program_analysis/tests/index.html

@@ -91,6 +91,10 @@ <h2 class="section-title" id="header-submodules">Sub-modules</h2>
 <dd>
 <div class="desc"></div>
 </dd>
+<dt><code class="name"><a title="skema.program_analysis.tests.test_import_cast" href="test_import_cast.html">skema.program_analysis.tests.test_import_cast</a></code></dt>
+<dd>
+<div class="desc"></div>
+</dd>
 <dt><code class="name"><a title="skema.program_analysis.tests.test_import_method" href="test_import_method.html">skema.program_analysis.tests.test_import_method</a></code></dt>
 <dd>
 <div class="desc"></div>
@@ -169,6 +173,7 @@ <h1>Index</h1>
 <li><code><a title="skema.program_analysis.tests.test_goto_basic" href="test_goto_basic.html">skema.program_analysis.tests.test_goto_basic</a></code></li>
 <li><code><a title="skema.program_analysis.tests.test_goto_computed" href="test_goto_computed.html">skema.program_analysis.tests.test_goto_computed</a></code></li>
 <li><code><a title="skema.program_analysis.tests.test_identifier" href="test_identifier.html">skema.program_analysis.tests.test_identifier</a></code></li>
+<li><code><a title="skema.program_analysis.tests.test_import_cast" href="test_import_cast.html">skema.program_analysis.tests.test_import_cast</a></code></li>
 <li><code><a title="skema.program_analysis.tests.test_import_method" href="test_import_method.html">skema.program_analysis.tests.test_import_method</a></code></li>
 <li><code><a title="skema.program_analysis.tests.test_literal_returns" href="test_literal_returns.html">skema.program_analysis.tests.test_literal_returns</a></code></li>
 <li><code><a title="skema.program_analysis.tests.test_loops" href="test_loops.html">skema.program_analysis.tests.test_loops</a></code></li>