Important changes

The ExprTK-based processes have had a first overhaul (not the last before 3.0 final).
have a look at https://ossia.github.io/score/features/exprtk.html for a bit of documentation on it.

Bugfix release

• Fix various crashes that could happen when opening / closing documents repeatedly.
• Mostly CI work
• Audio preview in the library should finally be working fine, please report any issue with it