Set and document default for exponents in set...() functions

[gustavdelius]

setSearchVolume() assumes that the search volume exponent q is given in the species_params data frame. This usually creates no problems because a value is set in the new....Params() functions. But that is not enough. validSpeciesParams() neither sets a default nor complains if q is missing. If one then calls setSearchVolume() with a supposedly valid species params data frame one gets a cryptic error. The straightforward solution is to let setSearchVolume() use a default value if q is missing. This default should be lambda - 2 + n

The exact same comment applies to setMaxIntakeRate() and the exponent n and also to setMetabolicRate() and the exponent p except that in the latter case surprisingly p can be supplied as an argument.