New modification statements

[kkaris]

This PR adds new Modification statement types:

• Carboxylation/Decarboxylation
• Amidation/Deamidation
• Sulfation/Desulfation
• Neddylation/Deneddylation
• Alkylation/Dealkylation
• Prenylation/Deprenylation

[kkaris]

I found some places that potentially need to be updated if the new types are integrated:

• https://github.com/sorgerlab/indra/blob/master/indra/assemblers/pysb/sites.py
• https://github.com/sorgerlab/indra/blob/master/indra/tests/test_english_assembler.py
• https://github.com/sorgerlab/indra/blob/master/indra/sources/ctd/processor.py

[cthoyt]

if you have a docstring, you actually don't need a pass

[cthoyt]

Maybe it makes sense to add a bit of metadata along with each statement type to align them with related entries in other controlled vocabularies. For example, neddylation corresponds to the GO biological process "protein neddylation" (GO:0045116).

I did this for PyBEL and you could copy relevant parts if you think this is a good idea. No single controlled vocabulary is covering all types of modifications, though. See: https://github.com/pybel/pybel/blob/57b24a9ddf6d5d73d82c92294ed2186ddf4443c5/src/pybel/language.py#L320-L524