Pick ancillary parameter(s)

[ellessenne]

Function in Mata to pick up ancillary parameters (depending on estimation command and distribution) can be factored out of std_surv() to prepare for compatibility with -stmixed-.

[ellessenne]

For now, process by hand -mestreg- and -stmixed- output. But could be extended to use internal -merlin- utility functions at some point.

[ellessenne]

See #17 – this may be redundant after that is implemented.