Modify atomic_data operators for fast interpolation

[marcosertoli]

Problem to solve
During multi-parameter optimisations, the FractionalAbundance and PowerLoss operators are called more than once and take up most of the execution time.

Proposed solution
Modify the classes such that if full_run == False the ionisation balance and power loss parameters are not calculated, but interpolated on the electron temperature profile in input. This is a typical solution used on various tokamaks, with the additional capability of creating the standard set of parameters with different starting assumptions depending on the need.

This requires minimal change to code in the classes and in their calls within the workflow.