Species dependent bakdif/fexp not consistently used

[David Dickinson]

Currently we allow the user to specify different bakdif and fexp values for each species in certain cases. In some cases we warn that this isn't appropriate (without other flags) and hence force all values to be the same, in other places we just use the values for one species for the treatment of all.

I propose we change this such that there is only one value for bakdif and fexp that is used for all species. I think the other alternative is to ensure that the species dependent values are used consistently everywhere.