Should the PTE solvers do something sensible for N=1 systems?

[Yurlungur]

This is a twofold question:

1. If I invoke the PTE solver for a pure cell, should it do something sensible? Should it just return?
2. If we create a reduced system for, e.g., the fixed T solver with only 2 materials, we could end up with a 1x1 matrix. We should probably do something sensible there.

Thanks @dholladay00 for pointing this out

[Yurlungur]

Partially resolved by #545 . Relevant for #546.