Make 1D interpolation function

[ole]

Create new callable object Interpolation_function_1D

for general use.

The constructor will take vectors x, y When called with an arbitrary value xi it will return the linear interpolation f(xi) based on the values x, y.

This should be used whenever the user needs to create a function based on a timeseries - e.g. from a hydrograph. However, this fundamental object should also replace internal functionality in the 2D Interpolation function, file_function, file_boundary or where appropriate.

[ole]

As a matter of fact, Interpolation_function already deals with pure time series, so this ticket might boil down to allow reading of csv time series - see FIXME i util.py in _file_function().

[ole]

One simple step towards facilitating reading of hydrographs is to make a csv2tms conversions routine.