change data loaders to 'inject' data instead of copying

[brendan-whelan-seetreat]

at the moment, we end up with two dataframes which is fine for smallish data, but quite inefficient for large data.
this could be avoided if data loaders 'inject' the data frame directly into the phase space (i.e. return it from a function)

[bwheelz36]

upon further consideration, maybe the copy step is not necessary in the first place...