devMode: Initial implementation

"devMode" is a state indicating that a score is still under
development. oll-files (or individual functions) can use this
state to visually highlight specific operations, presumably
through colors.

When including oll-base a variable 'dev-mode is initialized to #f
so any function can base a conditional expression on the boolean
value of 'dev-mode.
in-source-communication: Finish functions

\comment, \discuss, \todo, \followup

These functions implement devMode coloring.
isc-metadata
in-source-communication: update metadata
oll-doc: Improve header field parsing

Fixes two issues:
- header fields that are commented out are now ignored
- header fields with empty (one-line) string are read correctly
oll-doc: Catch exception of missing list fields

Field names for splitting the comma-separated fields are
hard-coded, so it causes an error when one of these fields
isn't present (or misnamed).
This commit catches this exception and produces a meaningful
message.
markup-utilities: Fix header name issues
in-source-communication: Add usage example
ignore-collisions: make devMode-aware
dev-mode: add -switch color
cross-voice: Add files (already updated)

Workaround to write cross-voice elements (curves, spanners ...)