Try to reduce the amount of code to represent hybrid electrons

This PR is primarily to hold discussion around the hybrid electron implementation. The code changes it introduces removes a lot of the special handling. This appears to lead to exactly identical response matrices to the original hybrid electron implementation, indicating that this approach may be equivalent. There remain some questions around possible differences this may introduce in the time advance, with reports of previous implementations seeing numerical issues after several restarts without the special handling in the current hybrid_electrons branch.

Set h_non_adiabatic to zero in the linear parallel solve (i.e. gnew = -adj). Remove special treatment in getan. Remove gamtot_hzero from field equations.

Whilst these changes successfully reproduce a linear collision scan test case very well need to test this further, particularly non-linearly.