[QCheck-STM] Improve heuristic for create/init function call generation

[n-osborne]

In order to avoid running out of memory while running the generated tests, an heuristic has been put in place to limit the size of the returned SUTs (see 5856e5c).

This is not always the behaviour that users want.

There are two possible solutions that I can see:

• provide a cli flag to disable the heuristic (see: [qcheck-stm] The generated tests use small_signed_int as int generators #257 (comment))
• make the heuristic smarter: we mostly want to limit the size of containers, which usually have a parametric type