Allow config['initial_conditions']=None option

[oliverwm1]

There are cases where a user might want to specify all initial conditions files via the patch_files option (e.g. if the initial conditions files need to be renamed or they are not all coming from the same directory). However if one leaves config['initial_conditions'] as default, the INPUT directory will be populated with irrelevant GFS initial conditions. The model won't use them if the namelist is set properly, but it's confusing.

[mcgibbon]

We discussed that you would use this change in a branch as a temporary measure. In the long run we want to do some more extensive changes, which we can discuss in #46.

[oliverwm1]

I found a workaround so I'm going to close this issue.