Add extra check to TE Graph to remove disabled or down Links before computing the solution

[italovalcy]

Fix #203

Heads-UP: this PR sits on top of #209

Description of the change

• This PR adds a simple fix to the topology TE graph to omit unhealthy Links (i.e., Links that are not UP/Enabled)
• Adding unit tests to cover requests and topology changes for disabling links

[coveralls]

Pull Request Test Coverage Report for Build 10350812753

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 86.164%

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Totals
Change from base Build 10350802503:	0.2%
Covered Lines:	1112
Relevant Lines:	1270

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💛 - Coveralls