Change csv to deal in strings and convert appended values into strings

[wfsyre]

Right now, the CSV writer class can only handle writing doubles to the csv table object. These get converted into strings when writing to the actual CSV. To simplify things, the CSV writer should just maintain a map of strings, which allows us to write any data type we want to the CSV as long as we convert it to a string first. It introduces some extra pre-processing we have to do to recover data from a CSV, but I think the schema of get() is already well established in scrimmage so its not a large burden. This also allows people the added control of determining how their data should be turned into a string instead of leaving it to the CSV writer to determine. I have left the scientific vs fixed schema in there but technically all that should be done before the CSV writer even sees the value. The Visitor stuff is a bit of a hacky way to leave some backward compatibility for the old Append method that only took a double.