[DOC/CI]: add flake8 and compliant code; update contributing - etc

Codecov Report

Attention: Patch coverage is 27.27273% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pvpltools/module_efficiency.py	0.00%	9 Missing :warning:
pvpltools/dataplusmeta.py	0.00%	4 Missing :warning:
pvpltools/iec61853.py	0.00%	3 Missing :warning:

Files with missing lines	Coverage Δ
pvpltools/power_conversion.py	`0.00% <ø> (ø)`
pvpltools/test/test_dataplusmeta.py	`94.91% <100.00%> (ø)`
pvpltools/test/test_iec61853.py	`100.00% <100.00%> (ø)`
pvpltools/iec61853.py	`0.00% <0.00%> (ø)`
pvpltools/dataplusmeta.py	`0.00% <0.00%> (ø)`
pvpltools/module_efficiency.py	`0.00% <0.00%> (ø)`