Implement Gsplines as Functions

[rafaelrojasmiliani]

Take the current implementation of PiecewiseFunction and rename it as Gspline which inherits from Function.

This overrides some method as as

PiecewiseFunction operator*(double); // multiply coefficients
PiecewiseFunction operator*=(double);
PiecewiseFunction operator+(const PiecewiseFunction&); // assert same domain, same partition, same basis, add vectors
PiecewiseFunction operator+=(const PiecewiseFunction&);