Add optional schematype tag that is auto-detected.

* Add <schematype> tag. If set to "flat", the flat schema is used.

* fix bug where custom user-set schema was being ignored.

* shorten and improve cli v flag help message
Merge branch 'auto-schema' of https://github.com/rwcarlsen/cyclus into rwcarlsen-auto-schema
Merge branch 'rwcarlsen-auto-schema' into develop