LALPulsar: point PyPI/Conda packages to use new Solar System ephemerides package

@matthew-pitkin has put together a new Solar System ephemerides package for distributing the Earth, Sun, and time standard ephemerides.

Once this package is available in both PyPI and Conda, the LALSuite/LALPulsar PyPI/Conda packages will not longer include ephemeris files, but instead will pull in the new Solar System ephemerides package. This should save ~18 MB from the PyPI package which hopefully helps alleviate package size constraints.

I imagine the Solar System ephemerides package should be an optional dependency of LALSuite, if possible, so that non-CW users don't need to pull in a large package they don't need. This may require updating of various how-to guides which use LALPulsar from PyPI/Conda.

To-dos:

• Get the Solar System ephemerides package on both PyPI and Conda.
• Update LALPulsar configuration options to allow not including any ephemerides (currently must include at least a minimum set): !2156 (merged)
• Update PyPI packaging, add tests to make sure ephemerides are still being found: !2156 (merged)
• Refactor fallback path passed to XLALFileResolvePathLong() to all per-library fallback paths (set at ./configure time) and both absolute and relative paths
• Update Conda packaging, add tests to make sure ephemerides are still being found

cc @matthew-pitkin @david-keitel