Warn about invalid apply_dimensions callbacks

[soxofaan]

from Open-EO/openeo-processes#471 (comment):

{
  "apply1": {
    "process_id": "apply_dimension"
    "arguments": {
      "data": {"from_node": "load1"},
      "dimension": "t",
      "process": {
        "process_graph": {
          "max1": {
            "arguments": {"data": {"from_parameter": "data"}},
            "process_id": "max",
            "result": true
          }
        }
      }
    },
  }

This is invalid openEO process graph: process arg in apply_dimension should be a callback that takes an array and returns an array, but here (max) it is a callback that takes an array and returns a scalar.

VITO backend still accepts this, probably by automatically wrapping the scalar in an array.
Keeping this behavior is fine for now I guess, but at least we should log a warning about this.