change_tv() should be able to add tv parameters when there are none

[kellijohnson-NOAA]

The Introduction vignette as written makes me think that change_tv() doesn't declare environmental linkages for parameters. It currently says

ss3sim/vignettes/introduction.Rmd

Line 363 in eeb0b33

Currently, the `change_tv` function only works to add time-varying properties to a time-invariant parameter. It cannot alter the properties of parameters that already vary with time. Also, it will not work with custom environmental linkages. Environmental linkages for all parameters in the OM must be declared by a single line i.e., `0 #_custom_mg-env_setup (0/1)` prior to using `change_tv`. Additionally, SS does not allow more than one stock recruit parameter to vary with time. Therefore, if the `.ctl` file already has a stock recruit parameter that varies with time and you try to implement another, the function will fail.

I would hope that this is done automatically, but I need to double check.

[k-doering-NOAA]

I think the environmental linkage is declared, it is just hard coded to always use link option 2:

ss3sim/R/change_tv.r

Lines 221 to 235 in 0d50a0e

	# Set the environmental link and environmental var (8th element)
	# where a 3 digit value is put in to specify link and env var. 2 is used
	# for an additive link. (e.g., to sepecify an additive link with env. var 3,
	# put 203 as the 8th element.)
	# link = 2 use an additive fxn of environmental variable (g)
	# value of g in year y (env(y,-g))
	# param`(y) = param + link*env(y,-g)
	val[8] <- as.numeric(paste0("2", "0", dat.varnum.counter))
	ss3.ctl[par.ch] <- paste(c(val, "#", names(temp.data)[i]), collapse = " ")
	dat <- data.frame("Yr" = ss3.dat$styr:ss3.dat$endyr,
	"Variable" = dat.varnum.counter,
	"Value" = temp.data[i])
	colnames(dat) <- c("Yr", "Variable", "Value")
	ss3.dat.tbl <- rbind(ss3.dat.tbl, dat)
	}

change_tv does need some refactoring, as it is still using grep statements rather than the list element names when read in the r4ss read/write functions.