Allow for returning the sensitivity coefficients of interp1d_unc()

[BjoernLudwigPTB]

For some cases of complex interpolations we need the sensitivity coefficients from the uncertainty formulae to calculate the tridiagonal matrix representing the uncertainties for the (correlated) interpolated real and imaginary measurement values.

In GUM_DFT we implemented this already with a parameter returnC, which we should do here as well.