Allow fixed dimensions to not be chunked

[forman]

Is your feature request related to a problem? Please describe.

There is currently no way to configure a dimension of a variable to not be chunked, except for the following two cases:

1. Users know the size of a dimension in advance, so they configure chunks accordingly.
2. Users know that a given slice dataset is not chunked in a given dimension, so they do not configure its chunks at all.

If the sizes of some fixed dimensions are unknown, e.g., their slice source uses a bbox parameter to spatially subset in x and y dimensions, this becomes an issue.

Describe the solution you'd like

Allow a chunk size to be None for a given dimension, which means, its chunk size equals its dimension size. This should only be allowed for fixed dimensions for obvious reasons.