Start writing functions

The aim of this pull request is to start some discussion.

1. opinions on docstring formats?
2. data formats - is everyone happy to use a Quantity array to contain the inter-spike intervals, for example?
3. warnings and NaN - coefficient of variation of inter-spike interval needs at least two spikes. If there is a single spike in the train, I have chosen to return NaN, but should we raise an Exception, or at least a warning?
Agreed, but I think it makes the code clearer by aligning the `return` statements.