Obsolete error message for GenericStorage?

[jokochems]

When I worked on #1007 and looked into the Coveralls output, I detected an Error message for the GenericStorage class to be uncovered. It can be found here.

I think it is outdated anyways and instead, we want the Investment object to be passed to nominal_storage_capacity. Thus, in my understanding, it could be safely removed. What do you think?

[p-snft]

You are right, the warning is unreachable now.