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 beginning. 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.
make-subexpressions-optional in 7 minutes and 45 seconds (queued for 2 seconds)6 jobs for