MEG: maxfilt: new trans options

Old behaviour:
• “transform”, if empty, then no trans will occur (as before)
• “transform”, if one number, then it refers to session number. 1 is
identical with “trans first”. So e.g. if you want to register to middle
session, then you can set it to 2 (in case of 3 sessions).
• “transform”, if 0, then it refers to “default”
New options:
• “transform” can be a path to a fif file
• “transform” can have multiple entries:
o Numeric array: e.g. [1 0], means “trans first” then “trans default”
o Cell array: if you want to mix numeric input with path to a file.