Using preallocated arrays as much as possible

[f-fathurrahman]

List of some possible cases:

• Calculation of Rhoe:

  function calc_rhoe!(Ham::Hamiltonian, psiks::BlochWavefunc, Rhoe::Array{Float64,2})
    # Rhoe will be updated with new calculated Rhoe
  end

• Diagonalization routines: all work arrays are collected into one type, say:

  mutable struct DavidsonWorkArrays
    .... # list of work arrays
  end

  An instance of this type can then be passed to diag_davidson!