Generate input files for CASAL

[nokome]

As a test of consistency of this model with an assessment model, generate input files that CASAL can be fitted to to ensure it estimates consistent parameter values (when given error free data). Data sets to simulate:

• catch history
• catch-at-age: ~20 years, by method, BLL, BT, 20+ group
• CPUE: LL 1990-2015

Turn off discard mortality. Due to length-based v age-based selectivity don't expect to get exactly the same results but should be "close enough".

[nokome]

This is implemented as part of the Model::generate_casal() function and there is a test that runs it.