nat_cst utility functions and constants

This package contains a few utility functions (see below), 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