refactor: Improved project code quality

[frgfm]

This PR introduces the following modifications:

• switched unittest to pytest
• improved documentation
• revamped setup
• added quality checks
• added contribution guidelines and code of conduct

[codecov]

Codecov Report

Merging #53 (cb2e087) into master (358edb6) will decrease coverage by 1.99%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   90.37%   88.37%   -2.00%     
==========================================
  Files          10       10              
  Lines         675      671       -4     
==========================================
- Hits          610      593      -17     
- Misses         65       78      +13     

Impacted Files	Coverage Δ
torchscan/process/memory.py	71.42% <0.00%> (ø)
torchscan/crawler.py	82.08% <100.00%> (ø)
torchscan/modules/flops.py	97.57% <100.00%> (-0.02%)	⬇️
torchscan/modules/macs.py	100.00% <100.00%> (ø)
torchscan/modules/memory.py	100.00% <100.00%> (ø)
torchscan/modules/receptive.py	64.86% <100.00%> (-35.14%)	⬇️
torchscan/utils.py	75.00% <100.00%> (ø)