Interpolate applycal

Created by: tammojan

This adds optional bilinear interpolation for ApplyCal, on frequency and time. Default is to keep doing nearest neighbor interpolation.

Linear interpolation could be very useful if some data is omitted during calibration.

The interpolation is on the real values in the H5Parm, so on either the amplitudes (where it makes sense) or on the phases (where linear interpolation doesn't necessarily makes sense).

I'm not sure if interpolating the weights is the right thing to do.