update to make use of validation of XML, and also rename of methods to make clear the existing functions operate on binding energies.

add logic to deal with reading in of files where the fitting is split over the positive and negative regimes.

added finctionality to deal with the passing of floats as x_values to the function, so it mimics the numpy.polynomial.polynomial.polyval signature.

created script for evaluating the value based on the polynomial coefficients, including if different parts of the data are fitted with different coefficents.

added updates to the polynomial data format, so when fitting is done to the positive and negative regions separately, then this can be writtenout to file. Made changes to reader to maintain back compatability with current data representation.

added tests for methods that do not output graphs, and updated methods so expected output is produced.

update function used- should subtract the solvation energies because you lose this energy during interaction of the two SSIPs with each other, since these are defined as energy change going from "vapour" to solution.