scipy.interpolate.interp1d may be removed in future scipy version

[JanWeldert]

We use from scipy.interpolate import interp1d in multiple parts of the code, namely

• https://github.com/icecube/pisa/blob/cfeb9b45a38f981059197c293f1a53aa5ab81a39/pisa/core/prior.py#L14
• https://github.com/icecube/pisa/blob/cfeb9b45a38f981059197c293f1a53aa5ab81a39/pisa/stages/background/atm_muons.py#L7
• https://github.com/icecube/pisa/blob/cfeb9b45a38f981059197c293f1a53aa5ab81a39/pisa/utils/spline_smooth.py#L10
• The new stage introduced in #https://github.com/icecube/pisa/pull/798

At the moment we are safe because we limit the scipy version, but in the future when we allow for newer versions of scipy this might become a problem.

[thehrh]

I'm finding a few more occurrences: https://github.com/search?q=repo%3Aicecube%2Fpisa%20interp1d&type=code

[thehrh]

Scipy docs suggest the following replacement for "broken-line" interpolation: https://docs.scipy.org/doc/scipy/tutorial/interpolate/1D.html#piecewise-linear-interpolation

[JanWeldert]

But in the numpy version you have to do the interpolation again every time you want to evaluate it.