fixes invalidindex error

[erwanp]

Description

replace .at() by .loc() . Fixes #427

[anandxkumar]

tested the same on my system, test_array 2 tests fails. One is caused by AssertionError: assert 'numpy' == 'cython' and other on my system says ModuleNotFoundError: No module named 'radis_cython_extensions' where as on Travis says value error which happens when radis_cython_extensions successfully gets imported. Due to the same radis_cython_extensions the 2 tests in test_examples fails.

Why locally pytest says ModuleNotFoundError: No module named 'radis_cython_extensions'. pip install -e .[dev] -v is not passing all the tests (missing dependencies like the above), it might need to be updated.

[erwanp]

Cython build failed recently, causing the errors above.

I found out it was because we specify Cython & numpy requirements both in https://github.com/radis/radis/blob/develop/pyproject.toml (the new python standard) as well as in the setup.py (setuptools standard)

Things will be smoother once Setuptools totally supports pyproject.toml (which is being worked on : pypa/setuptools#2970 )

Interesting read : https://stackoverflow.com/questions/64150719/how-to-write-a-minimally-working-pyproject-toml-file-that-can-install-packages

[codecov-commenter]

Codecov Report

Merging #428 (93c394b) into develop (8416766) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #428      +/-   ##
===========================================
- Coverage    77.10%   77.08%   -0.03%     
===========================================
  Files          161      161              
  Lines        18664    18664              
===========================================
- Hits         14391    14387       -4     
- Misses        4273     4277       +4     

[anandxkumar]

@erwanp the code quality check failed. I haven't seen error similar to the below:
The command "eval git fetch origin +refs/pull/428/merge: " failed 3 times.

[erwanp]

I think it failed because I merged before the end of the linting-tests. Physics/executing-tests are fine, we're all good !