fix globals and other NAMESPACE issues

[Ryo-N7]

na.omit is from {stats} and not base... used in plot_passnet()... tbh could just filter NAs in a different way using the imports we already have

otherwise the usual global vars shenanigans that i'll need to add into globals.R

[Ryo-N7]

but more importantly need to change all the @import to be more specific via importFrom but that might be its own issue to tackle separately

[harshkrishna17]

@import useful can be changed to @importFrom useful shift.column

[harshkrishna17]

The above change is in plot_passnet()

[Ryo-N7]

after discussion with Harsh, we can just throw the shift.column() function over into utils.R and modify it a bit better for our own use instead of importing the package just for that single function (which is written all in base anyways)