BUG on plot(FLSR) for SV models

[iagomosqueira]

The function to be ploted in panel 1 for the model fit uses as.list(model(x))[[3]] (R/plot.R line 312). For the SV model, like bevholtSV this depends on using our %+% operators, but these do not work when called isndie ggplot, as ssb is a numeric vector insteda of an FlQuant.

Possible solutions:

(1) Overload operators for all combinations of numeric and FLPar or FLQuant

(2) Redeisgn the implementation of SV models to work more cleanly with both inputs, maybe thorugh abPars()