[developers] MRS XML support

[Ann Copestake]

I have created an MRS dtd (similar to the RMRS one) and added support
for reading and outputting XML files.  The code is mainly in
mrs/basemrs.lisp - the dtd is in mrs/mrs.dtd 

>From an end-user's perspective, there are `Save as XML' buttons on MRS
display windows which will save the complete MRS as an xml file.  I
have verified that MRSs can be read back in to the extent that they
are mrs-equalp with a reasonable number of examples.  *.mrs files in
emacs have a Control-c m (if you've got .mrs set up as sgml-mode)
which will read in a mrs-xml that extends before and after the cursor
position and display a window.  Generation is supported from the
window.

There are some sample batch output functions in mrs/interface.lisp 

As usual, testing and bug reporting soon would be appreciated.
`Funny' characters in rel names etc are a possible issue in terms of
XML validity - we should perhaps test for these in the sem-i so that
users can be notified when the lexicon is validated.  I would leave
this up to Ben, however.

Ann