Add gamma functions to the math module

[soranjh]

Context:
The PR adds the gamma function and the incomplete gamma function needed for computing the Boys function to qml.math.

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #3495 (2ee7674) into qchem_jax_addmath (9a3f91d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           qchem_jax_addmath    #3495   +/-   ##
==================================================
  Coverage              99.87%   99.87%           
==================================================
  Files                    311      311           
  Lines                  27910    27926   +16     
==================================================
+ Hits                   27874    27890   +16     
  Misses                    36       36           

Impacted Files	Coverage Δ
pennylane/math/__init__.py	100.00% <ø> (ø)
pennylane/math/multi_dispatch.py	100.00% <100.00%> (ø)
pennylane/math/single_dispatch.py	99.71% <100.00%> (+<0.01%)	⬆️
pennylane/pauli/utils.py	100.00% <100.00%> (ø)
pennylane/qchem/integrals.py	100.00% <100.00%> (ø)
pennylane/qchem/observable_hf.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[rmoyard]

Thanks @soranjh for the changes 👍 It think that gammainc would be better a multi dispatch, as the behavior in single dispatch can be misleading. What do you think?

[rmoyard]

It looks good to me 💯 Could you add a unit test in the test_functions in the math module, nothing too elaborate, just make sure that it is working for different combinations of x and y (builtin, numpy autograd, jax). After that happy to approve!

[rmoyard]

Thanks @soranjh, it looks good 💯 There are some changes in the file `pauli/utils, is it intended?

[rmoyard]

Thanks @soranjh 💯