Choose calculation variant to calculate the code churn metric

[glato]

Hi @ishepard,

as propsed in #175 I have implemented another variant of the churn metric calculation, that can be configured in the initialization of the CodeChurn class:

def __init__(self, path_to_repo: str,
                 since=None,
                 to=None,
                 from_commit: str = None,
                 to_commit: str = None,
                 ignore_added_files=False,
                 add_deleted_lines_to_churn=False):

To ensure that this new calculation will not break anything, add_deleted_lines_to_churn is set to False per default.

The calculation variant switch is trivial:

if self.add_deleted_lines_to_churn:
        churn = modified_file.added_lines + modified_file.deleted_lines
else:
        churn = modified_file.added_lines - modified_file.deleted_lines

I've added the corresponding unit test test_with_add_deleted_lines_flag() in test_code_churn.py that proves the correct behavior of the new variant.

I've also added/modified a part of the documentation (processmetrics.rst) to include the new variant and the parameter.

I hope I haven't missed anything else.

[ishepard]

Thanks @glato for the amazing PR! Everything looks great!
You just need to fix some flake8 errors (just run pip install flake8 && flake8) and you will be able to reproduce.

Thanks!

[glato]

@ishepard Thanks for the hint, should be fine now.

[codecov]

Codecov Report

Merging #176 (ed25356) into master (0e6c225) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #176   +/-   ##
=======================================
  Coverage   98.32%   98.32%           
=======================================
  Files          15       15           
  Lines        1012     1015    +3     
=======================================
+ Hits          995      998    +3     
  Misses         17       17           

Impacted Files	Coverage Δ
pydriller/metrics/process/code_churn.py	100.00% <100.00%> (ø)