nmr star simple parser:  do sane escaping of double-quoted string values

 - special characters:  "  and  \

 - special characters must be escaped by \

 - no other escapes are allowed

 - all other characters are allowed within double-quoted value literals
simple unparser:  first hack attempt (uses locally modified state).
   - indentation: 2 spaces for save frames, 4 for datums/loops, 6 for loop body
   - all values surrounded in double quotes and escaped
   - identifiers preceded by underscore
   - empty lines between subunits
   - loops and save frames terminated by 'stop_' and 'save_', respectively

loop:  change data model to list of keys and list of values, to avoid
   repetition of the keys.  Number of values must be an integer multiple
   of number of values