nr_configure_pdcch() allocates the CCE, but this is not wanted, e.g., in
multi-user scheduling, since it might not be possible to allocate the
CCEs for these UEs. This commit makes the following changes:

- remove CCE allocation in nr_configure_pdcch()
- remove coreset search in nr_configure_pdcch()
- add functions get_coreset() and get_searchspace() to retrieve coresets
  and search spaces easily
- in nr_fill_nfapi_dl_pdu(), add extra parameters for aggregation level
  and CCE indices and the searchspace and coreset of a UE, which might
  be different for each UE
- allocate CCEs manually in phytest, RA, and DLSCH schedulers