Resolve https://github.com/dswah/pyGAM/issues/301

[jonathan-taylor]

• Instead of filling with zeros in _flatten_mesh and generate_X_grid create an X matrix with constant rows given by the midpoint of the edge_knots so they values will be valid.
  -Also centered output of partial_dependence as the Y-axis is meaningless.

[5ch0r5ch1]

works for me

[dswah]

thanks for this!!
im updating master to use the latest build dependencies...