Surface forcing file paths contain are setup for payu users

[ashjbarnes]

In the ERA5 example, the paths to each folder begin with INPUT as though the user were using payu. Better to just incorporate the inputdir that users provide when setting up the experiment.

Originally posted by @ashjbarnes in #79 (comment)

[navidcy]

yeap, let's do that or have payu as dependency of the package and explain what payu is briefly in the docs?

[navidcy]

@ashjbarnes, is this still the case?
I noticed that in the ERA5 example, use_payu is set to False (by default)

[ashjbarnes]

Yeah the use_payu is only turned on for the ACCESS example. To do this we'll just need a bit of logic to leave things as INPUT/ when payu is on, and inputdir when it's off