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.
6 jobs for make-subexpressions-optional in 7 minutes and 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #277940
cuda docker
build-documentation

00:00:47

passed #277939
cuda docker
flake8-lint

00:00:16

passed #277938
docker
minimal-conda

00:01:22

passed #277937
docker
minimal-ubuntu

00:01:07

passed #277936
win
minimal-windows

00:01:46

passed #277935
AVX cuda docker
tests-and-coverage

00:03:42