Add parsing of storage data when instantiating base grid

Currently, a base grid is instantiated using the `CSVReader` object, which reads CSVs for bus, plant, gencost, branch, and dcline. In the `TAMU` object, we automatically augment this with defaults for new storage entries, since we don't include any storage in the base grid. However, other grid models may want to include some amount of base storage, so we should have a way to read the relevant information from a CSV file (**storage.csv**?) and add storage devices to a fresh Grid object that way.