Feature/2dfunction

[whaeck]

This PR adds the following:

• a generic interface for a 2D function z = f(x,y). It does not define a domain like the 1D function did and currently only defined the evaluate() and call operator.
• the interpolation objects were updated to add 2D versions using functions
• the interpolation::Table was removed since it served no real purpose and the code was moved to the SingleTableBase class (this represents a base class for tabulated data using a single interpolation region)
• the name of FunctionBase was changed into OneDimensionalFunctionBase
• single interpolation region objects for 2D tabulated x,f(y) functions were added

The next PR will add the equivalent of InterpolationTable for x,f(y) data.