Fix Issue #102: Use the proper default value for `interpolate()`

• Due to changes introduced by MR !64 (merged), the default value of lp_degree of from_degree() constructor of the MultiIndexSet class is no longer None but the value stored in the global setting module. The default value of lp_degree in the high-level function interpolate() now conforms with the new default.

This MR should resolve Issue #102 (closed).