Only generate POs at end of atomic assignment.
Backported assignment contexts from symphony.
Initial support for operation call obligations.
Add explicit operation contexts at function entry point.
Add obligation for operation call when op has precondition.
Add mechanism for clearing stateful context from stack.
Initial attempt at post_op in contexts. Frames not used yet.
Remove unused vars.
Removed instanceof checks when generating post_op contexts.
Not used since we can use the GetOpCallVisitor to do the check.
Massive clean up of FIXMEs and TODOs.
Improve underlying keys of substitution mechanism.
Various fixes to substitution mechanism st AllExamples pass again.
Cleaner seperation between stateful and non-stateful contexts.
Thread usage of helper visitors through assistant factories.
Merge remote-tracking branch 'origin/assistant-rewrite' into newpog

Conflicts:
	core/pog/src/main/java/org/overture/pog/visitors/PogParamExpVisitor.java
Process module definitions directly with visitor in order to clear state.
Introduce fresh vars for wr variables in implicit ops.
Minor cleanups to operation calls.
Remove outdated FIXMEs.