Adding first version for Atlas::LoadProfile

I tried to base it on ActiveDocument and CSVDocument,
but both were inappropiate. Since this is something very
simple and flat, we can just as well use a YAML file.
Load profiles are part of the dataset
Added capacity_credit attribute to node.
Added parameters for wind capacity credit to area data.
Merge pull request #38 from quintel/lolp_calculation

Added load_profiles and capacity credit attributes to Atlas