band_gaps_he: add docstring to BandGaps, rename some of its options
band_gaps_he: add docstring to BandGaps, rename some of its options

- rename: fixed_eig_range -> fixed_freq_range,
  feps -> freq_eps, zeps -> zero_eps

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