Clean up dependencies

[munrojm]

• custodian, mpcontribs-client, boto3, and emmet-core[all] made optional dependencies
• AlloyRester import altered with warning to handle when pymatgen-analysis-alloys is not installed via emmet-core

[codecov-commenter]

Codecov Report

Merging #666 (f00c427) into main (9ec0a9d) will decrease coverage by 1.37%.
The diff coverage is 55.88%.

@@            Coverage Diff             @@
##             main     #666      +/-   ##
==========================================
- Coverage   87.10%   85.73%   -1.38%     
==========================================
  Files          37       37              
  Lines        1559     1619      +60     
==========================================
+ Hits         1358     1388      +30     
- Misses        201      231      +30     

Impacted Files	Coverage Δ
mp_api/client/routes/alloys.py	58.82% <ø> (-2.29%)	⬇️
mp_api/client/core/utils.py	57.69% <53.33%> (-29.81%)	⬇️
mp_api/client/client.py	68.55% <58.33%> (-0.57%)	⬇️
mp_api/client/routes/__init__.py	88.88% <60.00%> (-11.12%)	⬇️
mp_api/client/core/client.py	88.66% <100.00%> (-0.04%)	⬇️

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[munrojm]

@mkhorton and @tschaume, I have added these warnings on instantiation should boto3 and pymatgen-analysis-alloys not be installed:

 UserWarning: Alloy addon package not installed. To query alloy data install the pymatgen-analysis-alloys package.
 UserWarning: boto3 not installed. To query charge density data install the boto3 package.

I also raise an MPRestError with the get_charge_density_from_material_id method if it is called without having boto3.

Furthermore, I have added a custom __getattr__ method which is called when python cannot resolve an attribute (e.g. MPRester.alloys and MPRester.charge_density), and have included the same warning messages above as MPRestError exceptions.

Want to make sure we are on the same page before I merge anything. Let me know if you have any thoughts.

[tschaume]

@munrojm looks good overall. Left a few minor comments for your consideration.

[munrojm]

@mkhorton, I am going to merge this unless you have an issue with anything.