store drivers as struct, enforce float type

[juliasloan25]

Is your feature request related to a problem? Please describe. Currently, fluxnet drivers are stored in a NamedTuple (see here). It would be nice to have these in a struct instead. This would allow us to enforce the same float type for all of them.

We should also convert the float type of data we use to FT as it's read in - maybe in filter_column or transform_column. #501 includes a workaround for this, wrapping some drivers in FT before they're passed to Thermodynamics.jl. We should remove this once we correct the float type as we read in data.