SMT Updates/Improvements

[CB-quakemodel]

General improvements:

Add CEUS 2020 site term to ModifiableGMPE from toml parser + unit test and document it

Splitting tests for sammons and euclidean matrix plots + testing using not just median, now also 16th and 84th percentiles.

Expand unit test for GMC from XML (instead of from the toml) to include a third TRT. The third TRT's branchset contains GMMs already present in both of the other TRTs.

Using dictionaries for storing GMM LTs to make things more explicit and flexible elsewhere. For example:

• Ability to customise the labels used for GMM logic tree plotting (for example, we might want to labelled a given logic tree as the "updated GMC" or "original GMC" logic tree in the plots, rather than just "logic tree 1" or "logic tree 2"

• When plotting from an XML add the TRT of the LT in that file to the plot legends and exporting functions by overwriting the default labels in such instances