Segfault on instantiating mechanisms on empty regions (#1657)

More, subtler fallout of the ABI patchset: When mechanisms are added to empty regions, various, layered segfaults
appear.
1. `set_parameter` will receive a `nullptr` upon linear search, throwing `no such parameter`
2. Guarding this for finite `width` uncovers further need in the ABI interface methods
3. This is caused by early exit in `instantiate` on `width == 0`
4. Getting rid of this is better, as we can treat vacuous mechanism painting like all others...
5. ...but causes another segfault, since we default index arrays to `cv_pos.back()` but `cv_pos` is empty.

This patch fixes all of the above and in addition removes mechanisms without support before reifying them
on the FVM cells. Relevant tests are added.