Allow changing data type of EQLs Jacobian elements

[santisoler]

Description of the desired feature

We know that fitting an EQL gridder is a process that requires a fair amount of memory, mainly to store the Jacobian matrix.
One small step into reducing the required memory would be to change the data type used to store the Jacobian elements, which are defined as float64 by default.
Currently, the EQLs offer no option to change that, even though the jacobian method has an optional argument dtype to change it.
Would be nice to have a jac_dtype argument that could be passed to the EQL constructor in order to change the representation of the Jacobian elements.
Although the memory consumption won't drop by a lot, users may reduce it by half storing the elements as float32.
I suspect that using float32 won't significantly compromise the accuracy of the predictions.

Are you willing to help implement and maintain this feature? Yes