Add functionality to Ylm IO

[knelli2]

Proposed changes

This is the first of 3 PRs to allow reading horizon coefficients from a file into the shape map coefficients in a domain creator. This PR just factors out a function into a more accessible part of the code and adds the ability to read Ylm coefs at a specific time from a file.

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Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[wthrowe]

[optional] My understanding is that the goal of this function is that the output is effectively a prolonged version of the input strahlkorper to the requested l_max. If so, a simpler test for this part would be to check that fill_ylm_legend_and_data(strahkorper, larger_l_max) and fill_ylm_legend_and_data(Strahlkorper(larger_l_max, larger_l_max, strahkorper), larger_l_max) give the same result.

[wthrowe]

What is the expected use case where the caller doesn't know the exact time?

Mention that epsilon is a relative error, possibly by renaming it to indicate that.

[knelli2]

Say I've been dumping data every 0.5M and I want to read it it at time 123.5M. However, the actual time of the output is something like 123.499999999999748. The epsilon accounts for this. The rename sounds like a good idea though

[knelli2]

@wthrowe pushed fixups

[wthrowe]

OK. Squash and rebase.