Add Streamlit GUI

[edwardchalstrey1]

TODO:

• Think about where the dependency should go, e.g. should it be dev or a new "GUI" option in pyptoject.toml or maybe refactor these completely
• Update the Install section of the README accordingly and separate user/developer install
• Move the GUI section of the README to it's own section, not under Python API
• Make sure when you click any of the download buttons, it uses the current settings
• Also make sure the saved file has an appropriate name, maybe ask the user?
• Colour palette feature?
• Font picker
• Text size adjust
• Horizontal
  • Inverse xshift values
  • Rotate alllabels 90 degrees
• In horizontal mode, make the payoffs to the right of the terminal nodes instead of underneath (other labels should remain above edges)
• Text distance from edge
• GUI: switch between zoom/scroll and fit
• Information sets can be:
  • Filled (opacity)
  • Dotted lines
• Make horizontal checkbox a switch between horizontal and vertical
• Make sure the font settings apply to terminal nodes
• Bold seems to be broken
• Normalise Action Label Distance
  • Minimum should be current 2 (normalised to 0) not 1. Default should be midpoint current 3.5 (normalised to 0.5) and top current 5 (1)
• Add a node size option
• Tidy interface

Closes #63 because this PR moved pygambit into core requirements