Add forward functions for magnetic gradiometry components for dipoles

[santisoler]

Description of the desired feature:

Would be nice to include forward modelling functions for the magnetic gradiometry components due to dipoles. They would be the spatial derivatives of the components of the magnetic field, i.e.:

$$ B_{xx}, B_{yy}, B_{zz}, B_{xy}, B_{xz}, B_{yz} $$

where:

$$ B_{ij} = \frac{\partial B_{i}}{\partial j}, \quad i,j \in \{x, y, z\} $$

Are you willing to help implement and maintain this feature?

Totally!