gridly: Add format-cell-file-name function (WIP)

This is a preliminary function used to produce an alternative
file name for scores produced by \gridCompileCell.

TODO: Retrieve the number of segments in the grid and pad the
'segment' part of the file name with an appropriate number of zeroes.
(I don't know how to retrieve that number).

ly/gridly/__main__.ily

@@ -298,6 +298,12 @@ gridGetLyrics =
           'SequentialMusic
           'elements lyrics))))
 
+#(define (format-cell-file-name parser part segment)
+   (format "~a-~a-~a"
+     (ly:parser-output-name parser)
+     part
+     segment))
+
 gridCompileCell =
 #(define-void-function
    (parser location part segment)
@@ -341,7 +347,10 @@ gridCompileCell =
            (ly:book-process book
                             #{ \paper {} #}
                             #{ \layout {} #}
-                            (ly:parser-output-name parser))
+           (format-cell-file-name
+            parser
+            part
+            segment))
          #{ \setOption gridly.segment-range #cache-segment #}))))
 
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