ElectronicDensity enhancements

[mrossinek]

What is the expected enhancement?

The new ElectronicDensity property is very naive in its current form. It was intended to be a starting point for further improvements, some ideas I will be listing below.

• renormalization techniques based on expected quantities, for example particle number, spin, etc.
  • this may become particularly important once implementing algorithms iterating over the ElectronicDensity. The noise we incur when measuring these expectation values on a quantum device can hinder convergence of such algorithms significantly
• symmetry-recovery (similar problem as in the above)
• more initialization methods, for example:
  • based purely on number of spatial orbitals and particle number
  • ElectronicDensity.zero
  • ElectronicDensity.identity