Make kde_ebm pluggable

[noxtoby]

• I updated requirements.txt to install:
  • awkde from my fork of awkde (in case the original developer removes their repo)
  • pybind11 (for awkde)
  • kde_ebm from the official repo (making this model pluggable)
• I removed the (hard-coded) source code for awkde and kde_ebm (subfolders: mixture_model and plotting).
• I updated references to these modules accordingly throughout pySuStaIn code
• Updated README with new installation instructions

Tested by myself and @sea-shunned

[sea-shunned]

Further tested on different machines and OSs. All looks good, merging.