Add is_subgraph_isomorphic function

[georgios-ts]

Adds a function to determine if two graphs are induced subgraph isomorphic.
This new feature is simple to implement since it's the same code with is_isomorphic function except in feasibility rules we check >= relation instead of ==.

[coveralls]

Pull Request Test Coverage Report for Build 886373020

• 111 of 111 (100.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 96.799%

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Files with Coverage Reduction	New Missed Lines	%
src/iterators.rs	1	92.32%
src/lib.rs	1	99.33%
src/isomorphism.rs	4	97.44%

Totals
Change from base Build 885754244:	0.2%
Covered Lines:	8438
Relevant Lines:	8717

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💛 - Coveralls

[IvanIsCoding]

This is great, is_subgraph_isomorphic reuses almost all is_isomorphic code which in my opinion is an indicator of good design!

Just a minor debate: should we use a macro and pass $op, or should we go with a more standard approach and use an Ordering argument?

I am biased towards passing an argument, it feels closer to how we write our Python code.

[mtreinish]

This lgtm, just some issues with the docstring for the universal function and the default for id_order python is different from the rust functions we probably want it to be consistent.

[IvanIsCoding]

LGTM. The diff is larger using Ordering, but ultimately I believe it will be easier to maintain.