Add option to smoothly interpolate indices

[greglucas]

Right now, the indices are floored to the nearest 3-hour (Ap) or daily (F10.7) values. This means if you are doing orbit propagation and crossing one of these boundaries with input data suddenly jumping, the output atmospheric density can also have potentially undesirable step-change results.

This can be resolved by users themselves doing the interpolation and inputting the explicit values, but I think it might be nice to add a linear interpolator to ease this use-case. If someone wants a more advanced interpolator like a spline, that is probably something that should be done outside of the core pymsis package by the user themselves.

Here is a quick demonstration of these effects during the May 2024 storm.

The indices:

The resulting atmospheric density changes (~20%)