DataCube.apply_dimension should not have a default dimension

[soxofaan]

this default "t" for dimension arg in applu_dimension is overly opinionated (and apparently causing confusion about how the process handles dimensions):

openeo-python-client/openeo/rest/datacube.py

Lines 1366 to 1374 in 6da65a2

	def apply_dimension(
	self,
	code: Optional[str] = None,
	runtime=None,
	# TODO: drop None default of process (when `code` and `runtime` args can be dropped)
	process: Union[str, typing.Callable, UDF, PGNode] = None,
	version: Optional[str] = None,
	# TODO: dimension has no default (per spec)?
	dimension: str = "t",

DataCube.apply_dimension should align better with the official spec (required arg with no default)

[soxofaan]

created initial PR #775

however it might be better to first raise warnings to give users time to update their code

[soxofaan]

already added 948f928 to start showing warnings