let as.quitte support lists

[orichters]

• I need it to convert lists of magpie objects.

old:

data <- as.quitte(readGDX("fulldata.gdx",c("vm_Mport","vm_prodPe"), types = c("variables"),  field = "l" ))                                                                                Error in `UseMethod()`:
! no applicable method for 'as.quitte' applied to an object of class "list"
Run `rlang::last_trace()` to see where the error occurred.

new:

• it works

Feel free to improve in any way if needed.