Add FunctionFromDatafile() function to be used in input scripts

[JonathanMaes]

Several years ago, @JeroenMulkers created the FunctionFromDatafile function. This can be used to read a CSV file and create a function that maps any column to another column. I now added a test for this, and the feature can be merged.

The function is used as FunctionFromDatafile(filename, xColumnIdx, yColumnIdx, interpolation), where the available interpolation methods are the self-explanatory "linear", "nearest" or "step". Example usage:

f = FunctionFromDatafile("data.csv", 0, 1, "step")
B_ext = Vector(0, 0, f(t))

where the first column of data.csv (i.e. column index 0) in this example contains the timestamps, and the second column (index 1) contains the field magnitudes. Lines in data.csv starting with # are ignored, so can be used for e.g. a header row.