NF: routine for private section finding

Implements the algorithm given in section 7.8.1 of PS 3.5.  The
manufacturer puts a 'private creator tag' in tag nos 1-xFF to give the
section start of their private tag information.
TST: add wrapper check for slice normal test file

This file is also a Siemens mosaic file; test we detect this.
RF+TST: get_csa_header uses find_private_section

Refactor get_csa_header to use new find_private_section routine. Guard
against dictionaries being passed by tests by checking for private
section marker before checking full dcm_data for CSA.  Add tests for
this check.