implement lazy if branching

[soxofaan]

see:

• if: branching behavior? openeo-processes#246
• branching behavior and short circuit conditions #246 openeo-processes#257
• https://github.com/Open-EO/openeo-processes/blob/master/meta/implementation.md#branching-behavior

The if process should not blindly evaluate both the accept and reject value, but only the branch that is necessary.

This has now come up in a use case where user does something like this:

S1bands = if_(
  eq(s1_collection, "SENTINEL1_GRD"),
  S1bands.sar_backscatter().apply(lambda x: 10 * x.log(base=10)),
  S1bands.apply(lambda x: 10 * x.log(base=10))
)

With s1_collection == "S1_GRD_SIGMA0", the sar_backscatter process should not be applied, but I still get this error:
[400] Internal: Process "sar_backscatter" is not applicable for collection S1_GRD_SIGMA0_ASCENDING. (ref: 0817481a-7141-44e5-ac0a-1bcde5bcce3a)
And with s1_collection == "SENTINEL1_GRD", I get an error for no spatial filter. But the spatial filter is specified.
[400] MissingSpatialFilter: No spatial filter could be derived to load this collection: SENTINEL1_GRD . Please specify a bounding box, or polygons to define your area of interest. (ref: 0f888861-9cf5-4dce-876f-b43b6312169c)

[jdries]

is deployed