Don't use `testthat` inside of functions

[Aariq]

Bug Description

The testthat package is made for unit testing, not use inside of functions. testhat functions appear in at least a couple of places:

• check_ed_metheader()
• check_ed_metfile()

These should be replaced with calls to stopifnot() or something from the assert or assertthat packages

[Aariq]

Other places:

• PEcAn.atmosphere::check_met_input_file()
• PEcAn.atmosphere::check_unit()

[Aariq]

Partially fixed by #3016

[Sarthakaga15]

Hi @Aariq , can I please work on this issue?

[Aariq]

Hi @Aariq , can I please work on this issue?

Just be sure to look at #3067 so you're not duplicating efforts. I think most of the stragglers are being handled in that PR.

[moki1202]

@Sarthakaga15 there's one more instance that you can work on, in ./models/ed/R/write_ed_veg.R. if you need any further help on this, let me know.