nat_cst constants

This subpackage contains a few utility functions (see API), as well as the following natural constants, in cgs units:

Name	Meaning
c = 2.99792458e10	Speed of light
G = 6.67384e-8	Gravitational constant
kB = 1.3806488e-16	Boltzman constant
sigma = 5.670373e-5	Stefan-Boltzman constant
bar = 1e6	1 bar in cgs
atm = 1.01325e6	1 atmosphere in cgs
losch = 2.68676e19	Loschmidt constant
h = 6.62606957e-27	Planck constant
r_sun = 6.955e10	Solar radius
r_jup=7.1492e9	Jupiter equatorial radius
r_jup_mean=6.9911e9	Jupiter mean radius
m_jup = 1.89813e30	Jupiter mass
m_sun = 1.9891e33	Solar mass
AU = 1.49597871e13	Astronomical unit
pc = 3.08567758e18	Parsec
amu = 1.66053892e-24	Atomic mass unit
nA = 6.0221413e23	Avogadro constant
m_earth = 5.9722e27	Earth mass
r_earth = 637813660.	Earth radius