Allow scaled scenarios to disable energy sector

This comes with a number of negative consequences:

* Since we want to display energy-sector sliders in ETM, but nonetheless
  set the sector to have no demand, we can no longer use cached input
  starting values, but must re-calculate them when loading a local
  scenario.

* A specific exception is required in the DisableSectors plugin to set
  the energy sector to zero by setting number_of_units to zero, rather
  than setting demand to 0. This is because excess supply demand can
  overflow from decentral production back into the energy sector.
  Setting all the converters to demand=0.0 also caused the app to crash
  due to the stack being too deep while performing recursive-factor
  calculations. Reason unknown.

Ref #743.