Make generate_wcs_getcov_str function more generic

[charparr]

I don't think it makes sense to have individual args for var_coord and time_axis and slice_string
Rather, this function should be able to accept and unpack and arbitrary number of axes and slices.

data-api/generate_requests.py

Lines 90 to 99 in b26a247

	# if var_coord is specified, subsetting using the specific variable
	if var_coord is not None:
	var_subset_str = f"&SUBSET=varname({var_coord})"
	else:
	var_subset_str = ""
	if time_slice is not None:
	time_axis, slice_string = time_slice
	time_slice_str = f"&SUBSET={time_axis}({slice_string})"
	else:
	time_slice_str = ""