DMRS updates

[goodmami]

Create a new delphin.dmrs module/package.

• delphin.dmrs.DMRS class, which is like the current delphin.mrs.xmrs.Dmrs except there should be no conversion to Xmrs structures (it should be natively DMRS), and other irrelevant parts are removed
• delphin.dmrs.Node class, moved from delphin.mrs.components.Node
• delphin.dmrs.Link class, moved from delphin.mrs.components.Link
• delphin.dmrs.ICONSLink class (maybe? maybe rename?) which encodes ICONS information (see this thread: http://lists.delph-in.net/archives/developers/2018/002786.html)
• delphin.dmrs.dmrx moved from delphin.mrs.dmrx
• delphin.dmrs.simpledmrs moved from delphin.mrs.simpledmrs

Until v1.0.0, existing imports should not be broken if possible (see #139)

[goodmami]

More info:

• the DMRS class does not inherit from XMRS, but should be able to convert to and from it when necessary (e.g., for DMRS to MRS conversion). The other classes (MRS and EDS) will do this as well.
• the format-specific conversion methods to_dict(), from_dict(), to_triples(), and from_triples() will be moved to the format modules (e.g., delphin.dmrs.dmrsjson and delphin.dmrs.dmrspenman (and, again, likewise for other representations). The reasons for this are:
  • the classes were getting cluttered with conversion code
  • we may want different versions (e.g., dmrsrdf.to_triples() or something)
  • this way they can be proper codecs with load(), dump(), etc.
• DMRS-specific constants (e.g., EQ_POST, etc.) get moved to the delphin.dmrs package

[goodmami]

I moved the ICONS stuff to #220, so this issue is now complete.