Use new function locate_n2khab_data() for read functions

[florisvdh]

This PR removes the usage of fileman_up("n2khab_data") in providing a default basepath for the read functions.

Instead, this role is replaced by a dedicated function locate_n2khab_data() that:

• by default still largely does the same as fileman_up("n2khab_data"), but is not limited to the (default) 10 directory levels to search for the n2khab_data directory;
• takes into account the n2khab_data_path option (or if missing, its corresponding environment variable N2KHAB_DATA_PATH). If set, and on condition this filepath exists, the option takes priority and no search is done.