HDF format: use pytables internally instead of pandas

[alixdamman]

See discussion from issue #563

[alixdamman]

@gdementen have you already started something ?
I guess the idea is to have our own HDFStore, probably somewhat based on the one implemented in pandas ?

[gdementen]

Nope, didn't start something. In the long term, yes, but I meant first to only bypass pandas when saving/loading hdf stuff. This shouldn't be hard though.