Add plus sampling simulations

[LHMarshall]

Currently, simulations can only handle minus sampling designs.
This needs expanded to include plus sampling, this will help test edge effects.

[erex]

I believe dsims will perform minus sampling, but will give the wrong result, is that correct? I compared two simulations from the example provided in make.simulation help; one using minus sampling, and one using plus sampling:

plus.design <- make.design(region = region,
                      transect.type = "line",
                      edge.protocol= "plus",
                      design = "systematic",
                      samplers = 20,
                      design.angle = 0,
                      truncation = 50)
minus.design <- make.design(region = region,
                           transect.type = "line",
                           edge.protocol= "minus",
                           design = "systematic",
                           samplers = 20,
                           design.angle = 0,
                           truncation = 50)

The results suggests simulation in the presence of plus sampling might be doing the wrong thing, is that accurate?

minus.bias <- summary(minus.simulation.result)@individuals$D$percent.bias
print(minus.bias)
[1] 0.8600559
plus.bias <- summary(plus.simulation.result)@individuals$D$percent.bias
print(plus.bias)
[1] -20.04955

As an interim measure, could dsims generate an error or warning when make.simulation detects use of a design object that invokes plus sampling, rather than letting the user conduct a simulation producing an incorrect result?