Serve vector frames from THREDDS

First step has been to trim down `scripts/vector_animation.py` to `script/vector_frame.py`, which does nothing other than download the data, appropriately transform it.

It also changes the API of `mch_animation` (pending rename) to be more flexible about what when and what it downloads:
- Each vector frame can now be requested individually by index.
- Decimation factor is specified at class instantiation