Make subexpressions optional for constructing an AssignmentCollection

When introducing new people to pystencils it's often simpler not to
differentiate between `main_assignments` and `subexpressions` in the
Also for simple kernels subexpressions are often not needed, since
intermediate symbols can also be set in main_assignments.

Subexpression should be kept for expert users.
6 jobs for make-subexpressions-optional in 7 minutes and 45 seconds (queued for 2 seconds)