Remove `equals` and `scale` bounds

[Stefan Pfenninger]

Right now, variables can define equals and scale:

variables:
  energy_cap:
    foreach: [nodes, techs]
    where: "NOT run.mode=operate"
    bounds:
      min: energy_cap_min
      max: energy_cap_max
      equals: energy_cap_equals
      scale: energy_cap_scale

We want to remove the functionality for both of these.

equals can be handled by setting min and max to the same value, scale is rarely needed and can be handled separately (e.g. with an additional parameter) where it is needed.

Pyomo recommends the approach of setting min and max to the same value to fix variables:

https://pyomo.readthedocs.io/en/stable/working_models.html#fixing-variables-and-re-solving

[Stefan Pfenninger]

Done in #466