Updated Detectability

Can now create populations with multiple detection functions. Added
more validity checks on the Detectability object
catch ddf failures
suppress file not found warning
hazard rate

detection function in Detecability
record number of failed iterations
data check

check there are sufficient data points to fit a detection function
bug fix

wrong number of brackets
fix adj term bug

For some reason it doesn’t like calls in S4 objects, treats them as
formulas so there is a type mismatch. So have to make them formulas but
this requires constructing the ddf calls in a different way