Add methods to create surrogate datasets (eg, connectivity matrices)

+ It'd be nice to have methods to create a Connectiviity dtype (with all its attributes) from scratch.

+ TODO: elaborate more on this topic

• weights: random, adjacency, watts&strogatz, albert-barabasistrogatz, Erdős–Rényi mode. Use networkx, BCT.

• distances: x, y, z triplets could be drawn from points on a sphere or two hemispheres
• areas, orientations can be derived from the spherical surface.
• projection matrices: using openmeeg for single dipoles or considering the concentric spheres model

+ A few people have asked to have this feature to be able to generate smaller matrices.