Add fit_function functionality

[cdemulde]

In order to provide a better way of 'predicting' data, add a fit_function functionality, with as arguments an independent data series, a dependent data series and a function that can relate them to each other. See also the scipy.optimize.curve_fit function. Return values would be the function parameters.

Additionally, a fill_missing_function function should be provided, making use of the determined parameters and the given function to replace data.

The workflow for this is the same as when replacing data by means of correlations.