PoissonBracketCalculator operator

[gdgirard]

An operator that computes the Poisson bracket defined as $[F, G] = \boldsymbol{b} \cdot \left( \nabla F \times \nabla G \right) = (1/J_x) \epsilon^(ijk) \partial_i F \partial_j G b_k$ where F and G are arbitrary.

For more details, see article by V. Grandgirard CPC 2016 between Eq.4 & 5

Rk: input B_contravariant (compute norm_B inside)

After #18