Fix missing index in CSTR.equations

[xalelax]

I wasn't able to instantiate neuromancer.psl.nonautonomous.CSTR -- not even with default arguments; after some investigation, I found out that this is due to a missing index in the CSTR.equations method, which results in its output being

>>> [dCadt, dTdt]
[1.2090975763656697e-07, array([1.101448], dtype=float32)]

and this mix of a scalar and rank 0 array causes issues later on .cast calls in the cast_backend decorator.

I think this PR would also solve #229

[drgona]

thank you for this fix @xalelax - please rebase your PR from develop branch and we will be ready to merge