Add concatSegments.ily

Defines a \part variable concatenating segments (variables)
from 1 to 90 (in roman numerals).

Generated by a one-time utility script gen-concat-segments.py
Fix naming error in number-variables-roman.py

The script erroneously wrote

\IX = (etc.)
instead of
IX =
Fix naming error in gen-concat-segments.py

The script named the variable 'parts' instead of 'part'
Reapply Change marksAndBreaks_Roman to MultiMeasureRests

Unfortunately I had changed the spacer to MMR in 8b609d49d64e20a257954983e70923124c2bbdfe
in the target file marksAndBreaks_Roman.ily

So a new run of the script killed it :-(
Now I changed the source file marksAndBreaks.ily so the script generated the right dest file.
New version of marks-and-breaks.ly

Building on the previous commits we now have a structure part based on the new segments