Add invariant and constant diagonal shift at the same time in `QGTJacobian*`

Created by: attila-i-szabo

This PR allows specifying both a scale-invariant and a constant diagonal offset in QGTJacobian* at the same time.

@chrisrothUT and I discovered that adding a diagonal shift of the form diag_shift + diag_scale * S_ii is much more stable than pure scale-invariant shifting but remains faster and better converging than simply using diag_shift. To accommodate both, this PR deprecates rescale_shift and introduces diag_scale, the coefficient of S_ii in the shift.

Defaults:

• If nothing is specified, diag_shift=0.01, diag_scale=0.0 to recover the original behaviour
• If only diag_scale is specified, diag_shift=0.0
• If rescale_shift is specified, it behaves as it used to, but there is a deprecation warning
• Specifying rescale_shift and diag_scale together leads to an error

Internally, diag_scale is implemented the same way rescale_shift once was; diag_shift is added to it by adding offset=diag_shift/diag_scale to the scale factors used to rescale rows/columns of the S matrix.

There are some bits to iron out, most importantly whether the defaults above are good and diag_scale is a good name for the new parameter.