unify nomenclature for incidence-angle

[raphaelquast]

within the code, the use of theta_0 , theta_i and mu_0
is not fully consistent, evident from definitions such as:

• theta_i = np.arccos(mu_0)
• def thetap(self, thetai, thetas, phii, phis, a=[-1.,1.,1.])
  which is actually defined to represent thetap with zenith-angle arguments (theta_0, theta_s)

-> this needs to be corrected in order to be able to write a consistent documentation

[raphaelquast]

new nomenclature introduced in unified_nomenclature branch.

• tests and examples adapted and working

[bulli92]

branch was merged