Moving new release features to main

[lgrcia]

A lot of features in the core_paper branch (which is the version used in the paper) have to be incrementally integrated in the main branch. This issue keeps track of that.

• multi-precision core using mpmath (feat: add a multi-precision module for testing #210)
• making a CI pipeline producing an artifact to monitor the numerical precision of the main implementation (and speed)
• using the multi-precision $A_1$ matrix in starry light curve
• using the symmetry of the solution vector integrand for n>2 in order to speed up numerical integration (at least 30% speedup) (feat: precision helper functions #217)
• rotation matrices using s2fft (for now only a single edge-case grad test remains to be solved) (feat: rotation matrices using s2fft #212)
• order of numerical integration as keyword argument in starry light-curve (feat: order as kwarg and some ylm utilities + tests #197)
• Fixing starry light curves amplitudes of multiple bodies (feat: amplitude surface light curves for starry systems + tests #195)

[lgrcia]

Update: We are trying to merge #212 and #217 but encounter issues with tests on macOs (only). We think it's related to the use of s2fft I'm currently trying to pin down the issue.

[lgrcia]

All done in #227