Support xarray and numpy functions in expressions

[forman]

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

For example, we need the conditional function xr.where(cond, case1, case2) to compute new variables from xcube gen configurations and from expressions passed to xcube.core.evaluate.evaluate_dataset().

Describe the solution you'd like

Support the xr and np modules in expressions.

For example, allow the following:

• where(cond, case1, case2) (translates to --> xr.where(cond, case1, case2))
• xr.where(cond, case1, case2)
• np.where(cond, case1, case2)