More flexible src/rec coordinate definition

[prisae]

Make the coordinate definition for sources and receivers more flexible.

E.g.,

src = (
    0,  # x
    0,  # y
    0,  # z
    0,  # azm
    [0, 10, 20, 30, 40, 50, 60, 70, 80, 90]  # dip
)

[prisae]

For dipoles it is implemented in #185. For bipoles it is under way in #179.