Enhancements for runtime based slicing

[BrianMichell]

This will add the following features:

1. Support for runtime defined Dataset::sel() descriptors. Currently only Dataset::isel() allows for this to a limited capacity.
2. Remove hard compile-time limit for the number of runtime descriptors that can be passed. We can perform batch processing instead of erroring out.

Implementing 1. in a sensible manner may be complex due to templating, as dimension coordinates may be of different types.