Convert MinusZ model to Xija framework

Remove n_ccd=6 as a keyword input argument from the ConstraintTank class.
Add minusz_spec.json. This file definines the parameters for the xija version of the minusz model.
Replace nmass-based ConstraintMinusZ class with xija-based version.
Delete the import of the nmass module.

Signed-off-by: Matthew Dahmer <matthew.dahmer@gmail.com>