Throw away all HETATM lines. If only_strip_these_chains is specified then only strip HETATMs lines for those chains.

def strip_HETATMs(self, only_strip_these_chains = []):
        '''Throw away all HETATM lines. If only_strip_these_chains is specified then only strip HETATMs lines for those chains.'''
        if only_strip_these_chains:
            self.lines = [l for l in self.lines if not(l.startswith('HETATM')) or l[21] not in only_strip_these_chains]
        else:
            self.lines = [l for l in self.lines if not(l.startswith('HETATM'))]
        self._update_structure_lines()