Documentation and implementation of `runoff_file` and `alt_runoff`

[thurber]

Hello,

A postdoc user of Xanthos pointed out this issue to me. They were trying to reproduce the example run but with user-provided PET and RUNOFF files, but a few things conspire to prevent this:

1. The documentation here describes a runoff_file setting under the [Runoff] configuration tag. As far as I can see in the code, this setting is totally ignored. Instead, one needs to use the [Routing] [[mrtm]] alt_runoff setting.

2. Even so, if pet_module = none and runoff_module = none, neither the pet_file nor the alt_runoff file get passed on to the routing module (or the result files).

I will submit a Pull Request that can fix this issue for this particular use case, but I'm not as familiar with the broader context of the model so you may wish to address in a different way!