Get fishing base rate using a linear rate
getFishingBaseRate.linear(sp, fishing, T, ndt)
Current specie (sp0, sp1, etc.)
Fishing parameters
Number of years
Time step
It assumes that in the Osmose configuration, there is a "fishing" entry. It reads the rate.spX and rate.slope.spX parameters. The slope is computed any time the fishing frequency changes (fishingperiod parameter) For instance, if ndt=20, period=5, there are 4 time steps between two fishing time step. Therefore, the value will be (i0, i0, i0, i0, i1, i1, i1, i1, ...)