Model trainer: export model in tfjs format once trained

Dependencies

• [2] CWOSQTC4 Trust tensorflow's thread safety (it wasn't the cause of an earlier bug)
• [*] ZM2EMAZO Start doing python multi-module stuff properly
• [*] 7ML3OFE7 Model trainer: initial train and visualize thread pair (total crust mass; todo: altitude instead)
• [*] SJHJS463 Model trainer: main function to open sdl window and launch second thread for actual training
• [*] QBDHX7BH Add a makefile to express python dependencies since pipenv doesn't like tensorflow
• [*] VKA5CCGC Add pysdl2 to the trainmodel dependencies so I'll be able to visualize what's going on.

Change contents

• edit in trainmodel/src/__main__.py at line 5
  [5.29]

  [5.29]
  + import tensorflowjs as tfjs
• edit in trainmodel/src/__main__.py at line 14
  [2.146]

  [6.128]
  + tfjs.converters.save_keras_model(m, 'tfjs_model')
• edit in trainmodel/Makefile at line 10
  [7.165]

  [8.0]
  + env PATH=$(PWD)/venv/bin:$(PATH) pip install tensorflowjs