Improve flexibility of subcommands

[vanderhe]

Establishes two generalizations:

• allow for negative CAM-alpha and -beta values (necessary for SRSH-functionals)
• make maximum number of SCF steps and SCF convergence tolerance accessible from the user's perspective (skdef.hsd)

OnecenterParameters {

  C {
    .
    Calculator = SlaterAtom {
      MaxSCFIterations = 30       # default: 120
      SCFTolerance = 1e-08        # default: 1e-10 a.u.
      .
      .
      }
    }
  }
}

Builds upon #40, therefore ideally to be merged afterwards.
Closes #45.
Closes #46.

[aradi]

Very good idea! I am absolutely fine with this PR. Could you rebase and squash it, and then I'll merge.

[vanderhe]

Very good idea! I am absolutely fine with this PR. Could you rebase and squash it, and then I'll merge.

@aradi Thx, rebased on main and ready to go from my side. Shall I still squash the remaining two commits into one? Since they are dealing with two disjoint features.