add magic variable to get the number of notes inside a pattern

[truj]

Inside a pattern block it would be useful to know with how many notes the pattern has been called. This way we could use the same pattern for chords with a different number of notes.

[truj]

Additionally two more magic variables would make sense as well:

• one that contains all note indices in the given order
• one that contains all note indices in reverse order

These can be used for nested pattern calls with.

[truj]

Documentation is now available here: http://www.midica.org/tutorial-6.html#magic-variables