Make pyssht and healpy optional dependencies

[matt-graham]

Related to #229

Removes pyssht and healpy from required dependencies for s2fft as these are currently only used in tests and in JAX wrapper functions in s2fft.transforms.c_backend_spherical. Instead they are now added as test specific dependencies and the s2fft.transforms.c_backend_spherical module updated to try to import these modules when calling the wrapper functions and raise an exception with an informative error message instructing the user they need to install the relevant package to use the wrapper if not already installed.

As the dependency on pyssht is what was requiring pinning NumPy to <2 this resolves #266 as s2fft can then be installed with NumPy v2 or above if not installing pyssht, although here we still pin the NumPy version in the test dependencies so we are not testing against NumPy v2.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (876e090) to head (afdf08b).
Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #267      +/-   ##
==========================================
+ Coverage   96.28%   96.29%   +0.01%     
==========================================
  Files          32       32              
  Lines        3606     3619      +13     
==========================================
+ Hits         3472     3485      +13     
  Misses        134      134              

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[jasonmcewen]

This is very timely @matt-graham ! I was just today discussing this with someone who was very intersted in dropping some dependencies for ease of intergration in Euclid pipelines.

While removing ssht as a dependency is good for now, we should actually update ssht and so3 to support numpy>=2. This is discussed in this ssht issue and this so3 issue. It seems we just need to update PyPi to pick up new Conan recipes. Do you know how to do that @matt-graham?

[matt-graham]

While removing ssht as a dependency is good for now, we should actually update ssht and so3 to support numpy>=2. This is discussed in this ssht issue and this so3 issue. It seems we just need to update PyPi to pick up new Conan recipes. Do you know how to do that @matt-graham?

No sorry, I have not used Conan previously.

[matt-graham]

Discussed this in meeting today and conclusion was to merge this now without further review, so merging.