Idea: Rename `fxx` argument to `lead` or `step`

[blaylockbk]

I would like to rename the fxx argument to lead because that is easier to explain to someone what it means (it's the model lead time, after all, rather than explaining it as "the x-th forecast hour").

cfgrib uses the word "step", but I think "lead" is easier to understand and remember.

[blaylockbk]

Actually, I think step should be used instead of lead to match cfgrib. I don't want to use conflicting terminology from the package Herbie relys on.