Adding module PerHam3pla for 3-planet angles. Adding function formal_deriv_b in laplace module

I added a module called perham3pla containing a function PerHam3pla that essentially has the same behavior as PerHam but for 3-planet angles. These angles appear in the Hamiltonian at second order in mass of a Lie serie expansion after averaging out some or all 2-planet angles at first order in mass. I also added a function in laplace.py allowing the Laplace coefficients to be derived formally. That function is used by PerHam3pla