modfile.default() -- generate an `automatic' filename

default(root_name='undf', file_id='X', id1=1, id2=1, file_ext='')

This returns a generated file name, as (root_name)(file_id)(id1)(id2)(file_ext), where id1 and id2 are printed as 4-digit numbers, padded with zeroes if necessary. For example, '2ptn.B99990001.pdb' results from root_name = '2ptn', file_id = '.B', id1 = 9999, id2 = 1, and file_ext = '.pdb'. This mimics the `automatic' filename generation of previous versions of MODELLER when filenames were set to 'default' or '${DEFAULT}'.