enforce correct dtype promotion in qgt implementations

Created by: PhilipVinc

numpy.sqrt() was not a weak type and was promoting float32 neetworks to float64. This was an issue on QGTJacobian***

(This came up because I'm trying to run netket at single precision on gpu to get a speed bump and lower memory requirements).

@inailuig can I ask you to add a test for this?