pkmSetAICcTab: model names

[ddalthorp]

Define the model names in the return value so they match the format required as input in other functions.

In the return value of pkmSetAICcTab the model names are given in two separate columns: p formula and k formula. In other functions (e.g., plot.pkmSet) model names are written as one concatenated name, which makes it hard to pull out subsets of models from pkmSetAICcTab for use in other places.

For example, if I want to see diagnostic plots for the model with the best AIC or worst AIC or AIC's that are less than 3, I could do that readily if the model names in pkmSetAICcTab matched the format required in, say, plot.pkmSet.