Initial attempt at fixing ability to read TGA DTA .txt file

[OMWalmsley]

This PR attempts to solve parsing mp2028_281122_LNO+C.txt file.
Done by:

• Creating new TGA parser specifically for the .txt file.
• Writing tests for this new parser, that use the example file
• Integrating into the TGA datablock.
  • Allowing multifile selection for this TGA datablock.
  • Allowing partial pressures to be plotted from a .asc file with the temperature from the .txt file to also be plotted.
    • Current approach assumes that the time points from the respective files start from the same point but might not be the same length (as given in the example files) (see sub issue Aligning Timestamps on .asc and .txt #1899).
  • Allows the .txt DTA file to be plotted solely
• Fixing a bokeh issue where it fails to update the second plot line on a selectable_axes_plot graph.

Resolves issue #1897