Fix Python3 module, various GCC warnings and Makefile issues

[sdebionne]

This will probably requires further improvements to make it a Conda package.

• Improve build system to make it Conda compatible (e.g. use $CC and other env variables) rather than hardcoded values
• Write a conda recipe

[sdebionne]

For the records, the test that ships with the codebase fails with:

RuntimeError: could not integrate function

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/debionne/miniconda3/envs/lima/lib/python3.7/pdb.py", line 1697, in main
    pdb._runscript(mainpyfile)
  File "/home/debionne/miniconda3/envs/lima/lib/python3.7/pdb.py", line 1566, in _runscript
    self.run(statement)
  File "/home/debionne/miniconda3/envs/lima/lib/python3.7/bdb.py", line 585, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "/mnt/c/Users/debionne/Dev/pychooch/test_pychooch.py", line 1, in <module>
    import PyChooch
  File "/mnt/c/Users/debionne/Dev/pychooch/test_pychooch.py", line 11, in main
    result = PyChooch.calc(data, "Se", "K")
SystemError: <built-in function calc> returned a result with an error set

The GSL function that throws is gsl_integration_qag.

[beenje]

I missed that PR but basically did the same for Python 3 support in #2 + some update to latest chooch version.

The issue you mentioned was due to bug fixed in d08d636 upstream.

We created a local conda package at MAX IV. If there is interest, we could try to submit it to conda-forge.

[beenje]

chooch and pychooch are now on conda-forge!