Added a degree argument so can return the degree homogeneous part of the decomposition. oes not currently recurse.
Change degree code to give homogeneous components less than that degree not at given degree. Also changed so the default degree is None not -1.
Internal decomposition functions now always called with degree.
Add degree values so recursive step always calls with full degree.
Added a function to filter a variable decomposition.
Improved efficiency by only calling degree limited parts in recursion.
Added a negation in current version sets degree to default if it is set.