Forecast now function allow "plus an hour"

[jacobtomlinson]

Currently the forecast object has a method called now() which returns the current timestep from that forecast.

It would be good to be able to provide a datetime object as an argument to return different timesteps.

[jacobtomlinson]

This will probably require #2 and #3 to be completed first.

[Perseudonymous]

forecast.future() provides functionality to get the forecast for a point in the future, given by the number of days, hours etc. in the future.

I think the best thing to do is to have one function to return the forecast at a time in the future (a new function) and another to return the forecast some amount of time in the future (what forecast.future() does).