Vector container to hold component cubes

In response to the discussion on the google groups here
https://groups.google.com/forum/#!topic/scitools-iris-dev/xboZO5Oqifg
There was still a question about if this is a good approach.

Kept it relatively simple for now. Accepts 2 or 3 component cubes (of exactly the same grid) which are contained. Currently there is code for addition, subtraction, indexing, and the calculation of a vector magnitude cube. Can be relatively easily extended to direction, dot product etc, but I thought I'd keep it simple for the intial PR