Add 'RESTRICTION' to valid datatypes

[SimonGreenhill]

In the good old days, for certain analyses, MrBayes wanted the nexus datatype for a characters block to be 'RESTRICTION' (i.e. https://en.wikipedia.org/wiki/Restriction_fragment_length_polymorphism). This datatype is identical to datatype ='STANDARD'

[xrotwang]

Since the datatype is referred to from other places in the code, e.g.

commonnexus/src/commonnexus/blocks/characters.py

Lines 370 to 371 in 05b7b22

	if subcommand == 'DATATYPE' and self.datatype.upper() != 'STANDARD':
	self.symbols = []

I'm not sure just adding it as valid choice in one place is the right thing to do. Since it isn't mentioned in the nexus spec, I'd lean towards recommending to replace RESTRICTION with STANDARD in a pre-processing step.

[xrotwang]

see comment