Make `how()` easier to use

[gavinsimpson]

Right now you have to do

# 1
h <- with(data,
  how(plots = Plots(strata = f), blocks = block)
)

# or 2
h <- how(plots = Plots(strata = data$f), blocks = data$block)

I have been thinking for a while that allowing strata and blocks to take a formula and giving how() a new data argument would be a nice user-friendlyness update.

I'm thinking of

h <- how(plots = Plots(strata = ~ f), blocks = ~ block, data = data)

This would require data to be passed on the Plots() in some way.

Importantly, I will need to do this formula NSE myself - consult Thomas Lumley's Standard nonstandard evaluation rules document - as we can't have any Tidyverse code in permute as that would mean vegan would have a dependency on those packages too.