CRPropa 3.1.6

Bug fixes:

• Fix of (#254): Redshift evolution in PhotoPionProduction
  The reshift evolution was always handled with simple scaling and never with
  the more accurate 2 dimensional interpolation.

New features:

• New source feature SourceLambertDistributionOnSphere
  Simplifies simulations of scenarios with an initially isotropic and homogeneous
  distribution of cosmic rays on a sphere, e.g. for investigation of propagation
  of extagalactic cosmic rays in the Milky Way (see issue #246 and pull
  request #247)
• For PhotoPionProduction now a two dimensional interpolation of the redshift
  evolution is available (see pull request #255)
• A new break condition MinimumChargeNumber is added (see pull request #256)
• Vector3 now has index based access to its components. This improves
  interoperability with third party libraries, in particular with numpy arrays.
  (see pull request #262)
• Random seeds can be accessed from python (see pull request #263)
• New galactic magnetic field model by Terral & Ferriere (2017) (see pull request #258)
• Reimplementation of SOPHIA's photon field sampling used in
  PhotoPionProduction in c++, leading to a factor 2-3 speed up of the
  module (see pull request #260).
• Introducing a method, sophiaEvent(onProton, Eprimary, Ephoton), to
  directly call SOPHIA's event generator from python (see pull request #260).