created methods for solvation energy extraction, and set up test case, but tests need to be implemented.

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.