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.