More flexible src/rec definitions

[prisae]

#185 introduced more flexible definitions for the x/y-coordinates of dipoles (dipole, dipole_k, analytical).

This PR implements more flexibility for the definition of sources and receivers in bipoles (bipole, loop). Namely, that any of x0, x1, y0, y1, z0, z1/x, y, z, azm, dip can be provided of size (N,) or (1,).

Addressing #181