Remove extrapolation from interpolate

Needs more comprehensive treatment
Revert "spline methods no longer parametric"

This reverts commit ec8e2c11b6ba6b1a752383788e2578e994a5e29b.
Upd: parameterisation of interpolate & calibrate

Rely on Julia’s promotion system to deal with basic operations on
different types. However, need parametric methods here because x, y and
m may be vectors of different types of Real values.