Issue545 no udf found

[HansVRP]

No description provided.

[HansVRP]

PR to make the no UDP found error more informative:

OpenEoUdfException: No valid UDF found.

Signature errors:
  - Function 'apply_vectorcube' failed validation: Missing required parameter: 'geometries'

Found functions (none valid): apply_vectorcube

Expected signatures:
  - apply_timeseries(series, context)
  - apply_datacube(cube, context)
  - apply_hypercube(cube, context)
  - apply_vectorcube(geometries, cube, context)
  - Or a single-parameter function for generic UDF

[soxofaan]

I'm short on time, but some quick notes

if I understand correctly, this PR introduces quite some duplication between validation and execution of a UDF, which will be annoying to maintain in the long term.
The validation also doesn't seem to check type annotations, while the execution part does, so there is risk on inconsistencies here.

design-wise I would consider encapsulating everything in a new class, say UdfCodeParser, so that it is more clear that all these new functions belong together (most of which will be hidden internals), and I would try to unify the validation and execution

[HansVRP]

Does seem to become a large overhaul then; given the importance.

Maybe it is good enough to simply print a more informative error message which informs users of the supported UDF signatures?