Interpolation of data and coefficents

[oliverhaas]

1. Data interpolation is done with a "polint" function inside several modules, so there are duplicate "polint" functions. Better to move them into one place (probably an interpolate module).

2. In the same manner the actual interpolation and boundary handling is a duplicate in several modules, better move that to the base module.

3. Right now the end correction coefficients are loaded and then partly interpolated to the specific transform the user wants to do. The interpolation as of now is cubic, but it would be nice to use higher order for more accuracy if possible and in general not have another duplicate interpolation function (i.e. use "polint" function from point 1)