add csg.getFeatures(features) where eg features = [‘volume', 'area’]
function takes array of one or more features as an argument.

“getFeatures” can calculate any csg feature that is a (signed) sum of
tetrahedron features.
Currently only volume and area, but others (fourier coeff, moments) could be
added.

For performance reasons, the function is designed such that it can calculate multiple features in a single pass.
Merge pull request #38 from bebbi/getcsgfeatures

add csg.getFeatures(features) where eg features = [‘volume', 'area’]