Explicit errors if length_class and growth_parameters argument in multispecies case

[harryganz]

If the RVC object contains multiple species and the user uses the wrong syntax for the length_class or growth_parameters argument there should be an informative error returned. For example, if the user types

getStat(x, "domain", "density", length_class = "LM") 

Where x is an RVC object with mulriple species, he/she will get the error

Error in length_class[[spc]] : subscript out of bounds

This should be replaced with a more informative error.
Also, if the user types

getStat(x, "domain","biomass", growth_parameters = list(a=2e-5,b=3)

where x contains multiple species, he will get biomass estimates for all the species using the server estimates of the parameters without any warning that the input estimates were not used.