refactor nwp handling in pvnet.py

Detailed Description

Several functions in pvnet.py (find_valid_t0_times, slice_datasets_by_time, process_and_combine_datasets) have to unpack nested nwp sources when iterating through datasets and include preparation of specific parameters needed for nwp-specific functions. It might be better to take all of this out into separate functions and keep nwp datasets un-nested for this bit of code, to allow the functions to cleanly and homogeneously iterate over the sources

Context

Currently these three functions are a bit bulky and feel like they are doing a lot of stuff, this will compartmentalize them more.

Possible Implementation

something vaguely like this:

def is_nwp(key):
  split_key = key.split()
  return split_key[0] == "nwp"

def return_spacial_slice(da, config, key):
  if is_nwp(key):
    return spacial_slice_nwp(da, config, **get_additional_nwp_kwargs(da, config))
  else:
    return spacial_slice(da, config)

for key, dataset in datasets_dict.items():
  config = config.key
  slice = return_spacial_slice(dataset, config, key)
  slices.append(slice)