Change behaviour of comment argument of setter functions

[gustavdelius]

For example for setSearchVolume() the documentation of the comment argument currently says:

A string describing how the value for 'search_vol' was obtained. This is ignored if 'search_vol' is not supplied or already has a comment attribute.

This is not very practical, because one may well want to change an existing comment. So the behaviour should be changed to:

Optional. A string describing how the value for 'search_vol' was obtained. This is ignored if 'search_vol' is not supplied.

[gustavdelius]

Actually, I think we should just get rid of those comment arguments altogether and require people to put the comment directly on the value that they are setting. So instead of

params <- setSearchVolume(params, search_vol = my_search_vol, comment_search_vol = "my comment")

we'll have

comment(my_search_vol) <- "my comment"
params <- setSearchVolume(params, search_vol = my_search_vol)

We do however need to create a mechanism for going back to using the species_params. I propose that

params <- setSearchVolume(params, reset = TRUE)

should prompt a calculation of the search volume from the species parameters.