adding initial conditions for Oort cloud-like particle set and test

[jilkoval]

A class SphericalIsotropicCloud to create a isotropic Oort cloud-like particle set and a test:
"""
Spherical isotropic (Oort) cloud -- initial conditions for a particle set

See for example:

• Duncan, M.; Quinn, T.; Tremaine, S. -- http://adsabs.harvard.edu/abs/1987AJ.....94.1330D
• Feng, F.; Bailer-Jones, C. A. L. -- http://adsabs.harvard.edu/abs/2014MNRAS.442.3653F
• Dybczynski, P. A. -- http://adsabs.harvard.edu/abs/2002A%26A...396..283D

Initial conditions -- given by distributions of orbital elements:
semi-major axes -- power law, default: dn/da ~ a^(-1.5),
eccentricities -- dn/de ~ e,
constrain on the minimum pericenter,
isotropic orbits -- distribution of orbital inclinations: cos(i) = -1--1,
longitude of ascending node: 0--2pi,
argument of periastron: 0--2pi,
mean anomaly: 0--2pi,
equal mass particles.
"""

[ipelupessy]

two comments:
maybe put it in amuse/ic, and can you change the docstring style for the aruments to match:

:argument number_of_particles: Number of particles to include in the plummer sphere

[jilkoval]

done

[ipelupessy]

ok, also..can you add a factory function (like new_plummer_model)??

also you may find it useful that:

peri.value_in(units.AU)<min_peri.value_in(units.AU)

gives you the same as:

peri<min_peri