Rename the threshold property in ProcessInfo objects

[daanboer]

Currently CrossSectionInfo objects will contain a threshold property. This property should be interpreted as the energy that an electron loses as it partakes in the corresponding reaction. However, this value is not necessarily equal to the threshold of the (inelastic) process, as is often the case in processes involving compound products. Moreover, the name threshold is even more non-descriptive when regarding rate coefficients in heavy-species chemistry.

Therefore it is probably desirable to change the name of the threshold property in ProcessInfo objects. However, we should be very clear about its definition. I don't think the desired definition is currently completely clear, i.e. should it be the activation energy of the process, or the difference in formation energies between the produced and consumed species, or perhaps an entirely different definition. Needs discussion.