Move definition of priors to @estimate

[alistairdunn1]

Currently, priors are defined via a label in @estimate that links to a @prior command.

The definition of a prior needs to be moved to @estimate as a 'type', and the @prior command removed.

Plus, for each value in a vector that is estimated in @estimate, we need an element in a vector for the upper bounds, lower bounds, and any specific priors (like mu or cv for the lognormal prior), i.e., if estimating YCS for the years 2000-2004
@estimate
parameter process[MyRecruitment].YCS_values
prior lognormal

2000 2001 2002 2003 2004

upper_bound 10 10 10 10 10
lower_bound 0.1 0.1 0.1 0.1 0.1
mu 1 1 0.5 2 1
cv 1.1 1.1 1.1 1.1 1.1