band_gaps_he: update AcousticBandGapsApp to use compute_only engine option
band_gaps_he: update AcousticBandGapsApp to use compute_only engine option

- join coefs_basic + coefs_dispersion -> coefs
- update phase velocity computation
- update AcousticBandGapsApp.process_options(), .process_options_pv(),
  .call()

rc/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,lokik/sfepy,sfepy/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,vlukes/sfepy,sfepy/sfepy,BubuLK/sfepy,lokik/sfepy,lokik/sfepy,vlukes/sfepy,sfepy/sfepy,RexFuzzle/sfepy,vlukes/sfepy,rc/sfepy,lokik/sfepy,rc/sfepy,RexFuzzle/sfepy