#29 Multiple functor implementations

[Michael Stevens]

uBLAS functors are dispatched to multiple
implementations for dense/packed/sparse
iterator/containers.

Partly this is so that dense implementations can choose to
index rather then use the iterators!

With a generalised iterator infrastructure the iterators
should be able to take care of it. If indexing is more
efficient and the iterators can be indicies (rather then
pointers etc) then then only a subset of the functor
implementation is required