Feature constant detuning term

[lgiacome]

Description

Originally opened by @lgiacome in the gitlab repo: https://gitlab.cern.ch/IRIS/pywit/-/merge_requests/32

We implement the possibility to add a constant term (i.e. which doesn't vary when searching the instability threshold) to the detuning coefficients.
A new root-solving algorithm is also allowed (bisect) - now both newton and bisect may be used.
Includes several tests.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts

[nmounet]

Many thanks @lgiacome for the great work!
I just added the possibility to change the root-solving algorithm (newton or bisect) - both work in the tests.