oves(), ovets() - occurrence parts for ves() / vets()

[config-i1]

This is a followup thing based on Svetunkov&Boylan research. The idea is to model occurrence variables in groups.

Things to do:

1. Proper prediction intervals for ives / ivets;
2. Likelihood calculation;
3. Information criteria;
4. Automatic selection of the type of model in oves() / ovets().