Minor fixes to documentation of historical functions.
A bit of light tidying up and generalising of the tests.
Adding the contributions() function, which looks at the contribution per individual or per sub-community for a given diversity measure. It is normalised per sub-community, but there is a question mark over how the normalisation works for q = 1, where we are dealing with entropies. We do not do any further normalisation per individual, where the contribution is hard to normalise anyway, because we don't know the total population sizes.

Some tests are added to check that the raw and normalised contributions are the same when sub-communities are all the same size, and that they scale by number of communities when we move between the per sub-community contribution and the per individual contribution.