hard-coded dimension name in ImageCollectionClient.band()

[lforesta]

The band() method (which is a reduce over the spectral dimension) hard-codes "spectral_bands" as the dimension name for the spectral dimension of a collection. However, this info should be read from the collection metadata of the backend, since it may vary.
An option would be to send a request to /collections/collection_name when the load_collection() method is used in the client and store the collection metadata locally (temporarily, just for the current session).

Related to #25

[lforesta]

This issue is for the client, not the driver, sorry for the confusion. New issue here: Open-EO/openeo-python-client#93