added function for determinant_spline and corresponding test

[FSchwar]

Overview

Added a function to get the jacobian-determinant-spline of a given spline object.

Addressed issues

Showcase

A short / one-liner example to highlight the (new) feature

import splinepy
mySpline = splinepy.helpme.create.disk(3,1,180)
det_Spl = splinepy.helpme.create.determinant_spline(mySpline)

Checklists

• [x] Documentations are up-to-date.
• [ ] Added example(s)
• [x] Added test(s)

[j042]

please don't mark it resolved until it is resolved

[j042]

If this is finalized, could you please rebase?

[FSchwar]

Rebased with final improvements.