Cleans up CMakeLists & Python build scripts, fixes datetime string rendering

[sc1f]

This PR cleans up the Python build by removing vestigial code in CMakeLists and refactoring build scripts, as well as fixes to datetime handling:

• Streamlines CMakeLists & Python build/test scripts
  • Removes boost.filesystem from required dependencies
  • makes find_package(Boost) required for Python build
  • tests the selected python command before running, and defaults to python if the selected command (python3.7) etc. is not found
  • Refactors test_python.js
• Fixes Incorrect Values Rendered with uint8 Column on Datagrid #1084 - the full set of type interactions when updating Table with an arrow is now mapped out and tested.
• Replaces put_time with strftime using a 64-char string buffer
  • The maximum datestring that can be printed (10000-01-01 12:34:56:999999) is 27 chars, so this can be optimized a little further probably
  • if strftime fails, print the date in UTC using date.h as a fallthrough.

[timkpaine]

@sc1f this should probably go on top of #1057 or we're gonna have one hell of a conflict to clean up haha

[sc1f]

@texodus this branch is on top of the changes in #1077 - hence the large number of commits. 067f70c onwards are the unique changes

[texodus]

Looks good!