Add boxcar average option to M4i driver

[peendebak]

Changes proposed in this pull request:

• Add option for boxcar averaging
• Update class docstring
• Cleanup code by using the __transfer_buffer_numpy method

@WilliamHPNielsen @astafan8

[codecov]

Codecov Report

Merging #1509 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1509   +/-   ##
=======================================
  Coverage   73.81%   73.81%           
=======================================
  Files          92       92           
  Lines       10434    10434           
=======================================
  Hits         7702     7702           
  Misses       2732     2732

[codecov-io]

Codecov Report

Merging #1509 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1509   +/-   ##
======================================
  Coverage    73.8%   73.8%           
======================================
  Files          92      92           
  Lines       10444   10444           
======================================
  Hits         7708    7708           
  Misses       2736    2736

[astafan8]

Nice refactoring (apart from the new "features") that reduces code duplication!

[peendebak]

@astafan8 Addressed the comments. I am not sure about my solution to fix a mypy error (line 804). Do you have a better alternative?

[astafan8]

I'd suggest the following:

sample_ctype: Type
if bytes_per_sample == 2:
    sample_ctype = ct.c_int16
elif bytes_per_sample == 4:
    sample_ctype = ct.c_int32
else:

or a stricter version

sample_ctype: Union[Type[ct.c_int16], Type[ct.c_int32]]

Could you try them and see which one works?