Include support for LG4M and LG4X models

This update was suggested (and massively helped out) by Chris Laumer at
Harvard.

LG4 models are mixture models, which divide up sites based on their
rates in different ways. LG4M can be combined (in RAxML) with other
models in partitioned analyses, but LG4X can't.

LG4M has 1 parameter (gamma rate). LG4X has six free parameters - 3
rates and 3 weights (it doesn't use a gamma rate distribution).

Note that we can now fit partitioned models in partitionfinder that
can't actually be used in RAxML or PhyML, because we can e.g. specify a
model list that includes combinations of models that RAxML can't deal
with.
exclude LG4X models from all predefined model lists

this is to make sure that the default behaviour of PF is to only use
lists of models that can actaully be analysed in RAxML.
edit raxml parser to parse output from LG4* models
Include support for LG4M and LG4X models

This update was suggested (and massively helped out) by Chris Laumer at
Harvard.

LG4 models are mixture models, which divide up sites based on their
rates in different ways. LG4M can be combined (in RAxML) with other
models in partitioned analyses, but LG4X can't.

LG4M has 1 parameter (gamma rate). LG4X has six free parameters - 3
rates and 3 weights (it doesn't use a gamma rate distribution).

Note that we can now fit partitioned models in partitionfinder that
can't actually be used in RAxML or PhyML, because we can e.g. specify a
model list that includes combinations of models that RAxML can't deal
with.
exclude LG4X models from all predefined model lists

this is to make sure that the default behaviour of PF is to only use
lists of models that can actaully be analysed in RAxML.
edit raxml parser to parse output from LG4* models
Merge branch 'feature_LG4M' of https://github.com/brettc/partitionfinder into feature_LG4M