option to put AST columns in particular order

[lea-hagen]

Just heard from @benw1 that the PHAT artificial stars need to be in the order 475, 814, 275, 336, 110, 160. We should add something to the AST generating code (probably observationmodel/ast/make_ast_input_list.py), or perhaps in datamodel.py, to optionally specify the order. Or maybe just make a new script in tools to reorganize the columns.