Change "characteristic" to "intrinsic" variable.

All occurrences of "characteristic variable" should now be "intrinsic variable", and likewise "CV" becomes "IV".

Also renamed config strings for roles CVARG and CONSTARG to IVARG_ROLE and CONSTARG_ROLE.
Update documentation for the `itsdb` package.

Also renamed TsdbProfile to ItsdbProfile to be consistent (hopefully it doesn't break anyone's code).

The documentation for the API functions of `itsdb` are now mostly complete. There could be better descriptions of filters, key_filters, applicators, and data_specifiers, but this documentation is good enough to publish.
Move stuff out of mrs package docs.
Fix some serialization inconsistencies.

* Mrs(), Dmrs(), and Rmrs() functions are moved to delphin.mrs.xmrs
* build_graph() no longer takes `args` parameter, since the EPs need to have their own args so the ep.iv property works (Rmrs now converts EPs to MRS-style EPs, rather than the other way around)
* Mrs(), Dmrs(), and Rmrs() now take a Hook parameter instead of ltop and index (because ICONS may add more things, like ICONS-KEY and CLAUSE-KEY to Hook). The simplemrs unittests had to be updated for this change.
* A couple bug fixes with dmrx and eds serialization
* Renamed RSTR to RSTR_ROLE in config.py to be consistent
Remove LXML dependency; Add MRS documentation

The MRX codec still had the LXML dependency, but it's now removed. No LXML-specific functions were in use anymore.

Removed the classmethods from Xmrs, since the constructor methods Mrs(), Rmrs(), and Dmrs() are now in xmrs.py, and they do the same thing.

Also, added the mrs.components and mrs serialization documentation files.
Remove redundant exception and add helpful funcs.

Removed MrsDecodeError since XmrsDeserializationError does the same thing, but more generally. Besides, all codecs return Xmrs objects, even if they are meant for a particular *MRS variety.

Also added some useful functions to Xmrs (such as get_ep() and get_node()), the xmrs module (like find_argument_target()), and components.ElementaryPredication (get_arg()).
Add all variables to Xmrs graph to allow ICONS.

This puts in all variables (intrinsic, holes, etc) into the Xmrs graph, and adjusts query methods accordingly. It does not implement ICONS in serialization yet.
Add IndividualConstraint to documentation.

Also recases Rmrs constructor header.
Add ICONS support to SimpleMRS.

For v1.1, simplemrs will output ICONS if the list is not empty. If the list is empty, nothing is printed (unlike HCONS, currently, which still outputs HCONS: < >).
Fix outdated unit tests.
Add accessor functions to Xmrs and some docs.