gets the header for the .p4 file, note that this
       Advanced the file position to the end of the header.
       
       Returns the size of the header, and the size of the header,
       if the header keyword is true.

def get_header(file,**kw):
    '''gets the header for the .p4 file, note that this
       Advanced the file position to the end of the header.
       
       Returns the size of the header, and the size of the header,
       if the header keyword is true.
    '''
    if type(file) == str:
        #if called with a filename, recall with opened file.
        if test(kw, "gzip") and kw['gzip'] == 'guess':
            kw['gzip'] = re.search(r'\.gz$', file) is not None;
        if test(kw, "gzip"):
            with gzip.open(file,'rb') as f:
                return get_header(f,**kw);
        else:
            with open(file,'rb') as f:
                return get_header(f,**kw);
    if test(kw, "size"):
        size = file.tell();
    header = get_dict(
        file,'iiss',
        ['dump_type','dversion', 'title','revision']);
    if header['dump_type'] == 1:
        #this is a particle dump file
        d = get_dict(file,
            'fiiii',
            ['timestamp','geometry','sflagsx','sflagsy','sflagsz']);
        header.update(d);
        #reading params
        
        header['num_species'] = get_int(file);
        header['num_particles'] = get_int(file);
        nparams = get_int(file);
        units = [get_str(file) for i in range(nparams)];
        labels = ['q', 'x', 'y', 'z', 'ux', 'uy', 'uz']
        if nparams == 7:
            pass;
        elif nparams == 8 or nparams == 11:
            labels += ['H']
        elif nparams == 10 or nparams == 11:
            labels += ['xi', 'yi', 'zi'];
        else:
            raise NotImplementedError(
                'Not implemented for these number of parameters:{}.'.format(n));
        header['params'] = list(zip(labels,units));
    elif header['dump_type'] == 2 or header['dump_type'] == 3:
        #this is a fields file or a scalars file.
        d = get_dict(file,'fii',['timestamp','geometry','domains']);
        header.update(d);
        #reading quantities
        n = get_int(file);
        names=[get_str(file) for i in range(n)];
        units=[get_str(file) for i in range(n)];
        header['quantities'] = list(zip(names,units));
    elif header['dump_type'] == 6:
        #this is a particle movie file
        d = get_dict(file,
            'iiii',
            ['geometry','sflagsx','sflagsy','sflagsz']);
        header.update(d);
        #reading params
        n = get_int(file);
        flags=[bool(get_int(file)) for i in range(n)];
        units=[get_str(file) for i in range(n)];
        labels=['q','x','y','z','ux','uy','uz','E'];
        if n == 8:
            pass;
        elif n == 7:
            labels = labels[:-1];
        elif n == 11:
            labels+=['xi','yi','zi'];
        else:
            raise NotImplementedError(
                'Not implemented for these number of parameters:{}.'.format(n));
        header['params'] = [
            (label,unit) for (label,unit,flag) in zip(labels,units,flags) if flag
        ];
    elif header['dump_type'] == 10:
        #this is a particle extraction file:
        header['geometry'] = get_int(file);
        #reading quantities
        n = get_int(file);
        header['quantities'] = [get_str(file) for i in range(n)];
    else:
        raise ValueError('Unknown dump_type: {}'.format(header['dump_type']));
    if test(kw,'size'):
        return header, file.tell()-size;
    return header;