WIP: Add h5 cat clipper

[kejohnso]

This in-progress PR adds mostly simple changes to the cat tools in the MBTK that help in later parts of the workflow, plus a couple of bug fixes.

• adds a function for clipping catalogue to polygon before writing to csv/hmtk
• includes all the time fields (hr, min, sec) when exporting to hmtk
• changed magnitude conversion code so that it couldn't be used incorrectly (e.g. when conversion = "m") + a test
• fixing save fig to show legend
• in check for catalogue duplicates, adding minimum magnitude in subcat to avoid problems with binning when M<1
• fixing the magnitude precision - should check if this is still problematic following the other fixes
• preparing the catalogue for adding error to final magnitude
• adds function "process_magnitudes_keep_all" which keeps the magnitudes converted by the various equations for the same event so that they can be compared among each other
• adds a function to plot slab2pt0 profiles using matplotlib if the user is without pygmt

[kejohnso]

To-do in next phase:

• tests for purge_earthquakes.py
• adding std dev to magnitudes