Accepts any grid size

• First open-source version.
• Include RTD, Travis, Coveralls, Codacy, and Zenodo. No benchmarks yet.
• Accepts now any grid size (warns if a bad grid size for MG is provided).
• Coarsens now to the lowest level of each dimension, not only to the coarsest level of the smallest dimension.
• Combined restrict_rx, restrict_ry, and restrict_rz to restrict.
• Improve speed by passing pre-allocated arrays to jitted functions.
• Store res_y, res_z and corresponding eta_y, eta_z only if res_y, res_z were provided in initial call to utils.model.
• Change zeta to v_mu_r.
• Include rudimentary TensorMesh-class in utils; removes hard dependency on discretize.
• Bugfix: Take a provided efield into account; don't return if provided.