Tidied up the files involved in building a standalone version of ERS, adding a Makefile.standalone

  Tried to set up a way of building Ers simply on systems without a DRAMA
    installation. Created a new Standalone directory. In this I generated the
    two messgen-generated files (using the latest DRAMA messgen), Ers_Err.h and
    Ers_Err_msgt.h,together with a CONTENTS file saying more or less what this
    says here. At top level I added a Makefile.standalone file.

    It should be possible to build a standalone version of Ers using
                   "make -f Makefile.standalone"
    - this doesn't release any files, but it builds a libers.a.

    This has now been tested under OS X (High Sierra) and Linux (on aaolxp)

    Note that if anyone decides to define a new ERS error code in Ers_Err.msg,
    they should regenerate the various messgen-created files in Standalone:

       cd Standalone
       messgen -c -o -t ../Ers_Err.msg

   ACMM version 3.19.