Add satellite, cable, and whitehead_double functions

[alexeldridge]

This change adds several functions to perform satellite operations on knots. Given an (n, n)-Tangle representing a pattern knot in the solid torus, the satellite function returns the satellite of a given knot by that pattern optionally with a specified number of full twists. The satellite inherits its orientation from the pattern. Specialized functions for cables and Whitehead doubles are also implemented.

[NathanDunfield]

@alexeldridge Many thanks for doing this. Overall the code looks good, but I have two requests before merging:

1. I think most of your new functions should be made methods of Link. You can do this in spherogram_src/links/links_base in a similar matter as Link.backtrack.

2. The new methods should have doctests in their docstrings. These both give examples for the user and can serve as regression tests (e.g. you could check the number of crossings of some cable, or compute tau of some whitehead double). To make sure they pass, install spherogram and then do python -m spherogram.test.

Let me know if anything is unclear.

[alexeldridge]

I've added the methods to links_base.Link along with some doctests as requested.

One thing I noticed while testing some edge cases: the __repr__ for Link includes a count of link components, but does not include self.unlinked_unknot_components. I went to change this to include all components, but this breaks a couple doctests. Would this change be helpful (changing the appropriate doctests too) or is the current behavior intended?

[NathanDunfield]

@alexeldridge Thanks for the changes. I made few changes to the doctests to be compatible the auto-generator for the documentation for SnapPy, which uses reStructuredText rather than Markdown.

Concerning __repr__: unfortunately, the handling of components with no crossings is a bit of a mess, of which this one aspect. Let's leave it be for now.

I'll merge this PR shortly.

[NathanDunfield]

Ok, merged and I put a note in SnapPy docs to mention this in the new features list for the next release. Thanks for contributing this!