Simple input

* tweaks to XMLFileLoader to allow subclassing
* subclass to handle xml flat schema format
* added cyclus-flat.rng.in
* refactored recipe creating method into its own function for reuse by other things in future like recipe libraries.
* added cli flag for toggling to flat schema format
* added cli flag for user specified schema file
* fixed some missing details regarding cloning/initing

Companion PR in cycamore cyclus/cycamore#175