New function to convert isoscapes from sources to tissues

[courtiol]

For assignments, IsoriX converts the isotopic signature of the samples to assign (e.g. animal tissue) into the isotopic signature of the sources (e.g. precipitation), and not the other way around.

Yet, it can still be useful for people to convert the precipitation isoscapes into animal tissue isoscapes for the study of these isoscapes.

We would need for that a function that rescales a produced set of precipitation isoscapes; perhaps called:
rescalescape(), scalescape(), transcape(), isotranscape(), isoshift(), isoscale(), isorescale(), isorescalescape() ... ? (winner seems to be isotranscape())

Input should be: isoscape and calibfit.

Internally, we should call .invert_reg to invert the calibration fit and rescale the mean isoscape value using the obtained intercept and slope.

It is not immediately clear how to rescale the prediction variance and the residual variance since the rescaled isoscapes should ideally be influenced by the variances of the calibration fit and thus depend on which calibration method has been used.

Perhaps as a first step, we should only output the mean isoscape which is anyhow the one that users request.

The class of the output should be c("ISOSCAPE", "ISORESCALED", "list").
We should make sure that the object of class ISORESCALED cannot be used for assignment.
We should also detect this class and adapt things (title and isoscape not possible to draw) based on that class element.