add new SCF diagnostic quantities for slateratom

[alexmaryewski]

This is another one from the old draft.

This PR adds two new diagnostic quantities to help track the SCF convergence: the occupied part of the eigenvalue spectrum and the orthogonalised [F,PS] commutator. The former helps to spot if the orbital energies (one quantity that goes directly into the SK files) are converging properly, the latter is just a popular convergence metric that is used by many quantum chemistry codes out there (Turbomole, for one), and thus its implementation would make direct comparison easier. None of the changes add new or alter existing functionality, but they are to help with further debugging whenever new extension to the code are made.

I'm not sure about the output format. In this PR, the new quantities take place of energy components in the output loop, like this:

  Iter |   Total energy  |  d(Spectrum)  |   d(Commutator)   |  d(Potential)
 --------------------------------------------------------------------------
   1   -0.457372080E+02  0.936361041E+01  0.698559924E+01    0.997865497E+01
  
   2   -0.542210337E+02  0.420185025E+01  0.138941972E+01    0.576569651E+00
  
   3   -0.544037238E+02  0.355577050E+00  0.565883699E+00    0.584803006E+00
  
   4   -0.544171921E+02  0.536017478E+00  0.136724190E+00    0.180180852E+00
  
   5   -0.544192146E+02  0.169040510E+00  0.828489182E-02    0.543483765E-02
  
...

I'm very welcome to your suggestions as to where to make them available instead of this.