Support `intersection(::BSplineSpace, ::BSplineSpace)`?

[hyrodium]

In BasicBSpline, BSplineSpace (AbstractBSplineSpace) structs are regarded as a linear space, so we can compute their intersection and +.

I'm not sure we have practical benefits for the features.

[hyrodium]

+(::BSplineSpace, ::BSplineSpace) is not always BSplineSpace. Here are counter examples:

• BSplineSpace{2}(KnotVector([0,0,0,1,1,1])) + BSplineSpace{3}(KnotVector([2,2,2,2,3,3,3,3]))
• BSplineSpace{0}(KnotVector([0,1,2])) + BSplineSpace{3}(KnotVector([0,0,0,0,2,2,2,2]))