add option to calibrate Lorenzen M to a range of ages rather than single age #328
Description
The existing Lorenzen M formulation in SS3 requires the specification of M at a fixed reference age. This is difficult in situations where M is not known and is estimated from a maximum observed age based on assumed survivorship to this age. The current practice in the southeast is to calculate the Lorenzen curve from an external growth model and use this to calculate a reference age and associated M. When growth is estimated in SS3 this often results in an internal M vector that does not achieve the original survivorship assumption. To correct this we have developed an alternative Lorenzen M formulation (See associated pull request) that takes survivorship and an age range as inputs to calculate the M vector.