Move topology files from pce to datamodel

[sajith]

Fixes #103. Changes:

• Copies amlight.json, sax.json, and zaoxi.json from pce to src/sdx_datamodel/data/topologies so that they will included in the Python package.
• Also copies ampath.json and sdx.json to that location from test data directory for good measure, although I am not entirely sure of their usage -- they can perhaps be removed now or eventually?
• Copies test_request.json and test_request_no_node.json from pce to src/sdx_datamodel/data/requests. Also copies p2p.json to the same path, as test_request_p2p.json.
• Expands test coverage for topology/connection handlers and validators, so that some more cases are tested.
• Adds a tox configuration so that tests can be run with several versions of Python.
• Formats all concerned JSON files (with python3 -m json.tool <infile> <outfile>.

[coveralls]

Pull Request Test Coverage Report for Build 5859849091

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 74.635%

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Totals
Change from base Build 5800666242:	0.5%
Covered Lines:	1029
Relevant Lines:	1247

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💛 - Coveralls

[YufengXin]

sdx.json was the combined topology from the three domain topologies. I kept it for certain unitests related to pce because it works on the combined inter domain topology.

ampath.json might be a temporary file in early days. We probably could remove it later.