bug chart.forward(): missing units

When i create my own Symbol data in minutes, my `periodicity(R)$units` is ` 'mins' `.
as a result the function throws follwing Error:
```
Error in if (units == "months" && nrow(R) > 500) units <- "years" :
missing value where TRUE/FALSE needed
```
I would propose to add "mins" to the units (starting row 111 in chart.forward.R):
```
units <- switch(units,
mins="hours",
seconds = 'hours',
minutes = 'hours',
hours = 'days',
days = 'months',
weeks = 'years',
months = 'years',
quarters = 'years',
years = 'years')
```
I dont know if this counts as a Bug.

Open chart.forward.R around the units mapping starting near row 111, then reproduce the issue with Symbol data whose periodicity units are "mins". Extend the mapping so this input is handled consistently with the other short intervals, and confirm chart.forward() no longer throws the missing-value error.

Written by the indexing model from the issue text.