pm_SEprice assignment repeated unnecessarily all over the model.

[Renato-Rodrigues]

There is unnecessary code repetition for the definition of pm_SEprice in the model.

It is currently repeated in multiple places in the model. However, most probably only two assignments are really required: one at the core/preloop, and another in the core/postsolve.

Repeating the assignment at every power sector module realization, and at every presolve is quite unnecessary, extends the code length and can create potential oversight issues on any future change that can affect this parameter.