Replace _BlockData with BlockData (cog-imperial#144)

Pyomo recently made ComponentData classes public
(Pyomo/pyomo#3221) which will be part of the
upcoming release. Currently, this causes the following error to occur in
OMLT:

```
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
```

The Pyomo team is working to try to address this issue, however OMLT
should update its code to address this as otherwise deprecation warnings
will be emitted when using the old class names.

The fix is to replace all instances of `_BlockData` with `BlockData`
(just removing the underscore) - this applies to any other instance of
Pyomo component data objects as well (although I could only find 2
instances of these in the OMLT code).

**Legal Acknowledgement**\
By contributing to this software project, I agree my contributions are
submitted under the BSD license.
I represent I am authorized to make the contributions and grant the
license.
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Co-authored-by: jalving <jhjalving@gmail.com>

docs/api_doc/omlt.block.rst

@@ -8,7 +8,7 @@ OMLT Block
   :show-inheritance:
 
 .. note::
-	`OmltBlock` is the name used to declare the custom Pyomo block which is exposed to the user. The block functionality is given by `OmltBlockData` which inherits from Pyomo `_BlockData`.
+	`OmltBlock` is the name used to declare the custom Pyomo block which is exposed to the user. The block functionality is given by `OmltBlockData` which inherits from Pyomo `BlockData`.
 
 .. autoclass:: omlt.block.OmltBlockData
   :members:

src/omlt/block.py

@@ -25,11 +25,11 @@ class is used in combination with a formulation object to construct the
 """
 
 import pyomo.environ as pyo
-from pyomo.core.base.block import _BlockData, declare_custom_block
+from pyomo.core.base.block import BlockData, declare_custom_block
 
 
 @declare_custom_block(name="OmltBlock")
-class OmltBlockData(_BlockData):
+class OmltBlockData(BlockData):
     def __init__(self, component):
         super().__init__(component)
         self.__formulation = None