`fgen_interp1d` module

!40 (merged) identified that we're going to need tools to do interpolation.

I suggest making an fgen_interp1d module, which basically just copies the API of https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.interp1d.html but in a Fortran-like way, e.g. learning from https://stackoverflow.com/questions/73425771/spline-interpolation-in-fortran. That would give us a base class that basically had a method to initialise based on some data, then can be called as spline_instance % calculate(t) to calculate the value at a given time t (raising an error if t is outside the time bounds or not, cross-ref #21). I think this would be pretty simple and give us more or less all the power we needed.