Allow no inelastic collisions

[AndyMN]

Problem: When using an LXCat file with ONLY an elastic collision the solver will error out on the hstack function in _PQ. This is due to the fact that sometimes _PQ is called without setting the parameter reactions. This parameter will then be set to inelastic collisions, but no check has been made for the case when there are no inelastic collisions.

Following plots show the electron velocity W, reduced diffusion coeff, and mean energy of Maxwell type testsystem (constant momentum transfer collision frequency).

The labels are:
Maxwell = The changes proposed in this MR with an LXCat file containing ONLY the elastic collision cs
MaxwellAttachment = The changes proposed in this MR with an LXCat file containing BOTH elastic collision cs AND 0 attachment cs to add a non participating inelastic process.
MaxwellOld = The previous version with an LXCat file containing BOTH elastic collision cs AND 0 attachment cs to add a non participating inelastic process.

The results show that all 3 methods produce the exact same results so that the proposed changes in this MR do not change results, but only adds flexibility by allowing the use of a single elastic cs and no inelastic cs.