GridVoxel::calculate_emissivity_and_opacity(): add factor accounting for stimulated emission to total opacity chi
EOS: changed most arguments to from "const" to "const &" so that we don't have to copy so much memory

This should speed up the calculation of calc_LTE_populations especially.
EOS: changed args of Phi_*() functions from "const" to "const &"
main(): changed OpenMP decomposition of opacity calculation from GridVoxels to GridWavelengthPoints

The number of wavelength points will dwarf the number of grid voxels. So if we
decompose the opacity calculation among the voxels, we may end up with more
threads than voxels, and those extra threads will just be spinning idly.
main(): added another endl after status update about reading model data
main(): added status update about building wavelength grid
globals: added vector of Atoms into which atomic data will be read only once
Phi: fixed forgotten cast of statistical weight from unsigned int to double

This caused a lot of LTE level populations to be zero or NaN due to
decimal truncation.
Merge branch 'develop' into read_atomic_data_only_once
Atom::Atom(): converted atomic_weight from molar mass to atomic mass
Doppler_width: changed electron mass to atomic mass

Error in math. Using electron mass in calculating Doppler widths of
lines was making them way too broad. Should be the mass of the emitting
particle, i.e., the atom.
planck_function(): moved from main src dir to misc
misc: added function to calculate temperature derivative of Planck function
constants: added radiation-density constant
grid: fixed a bunch of sign errors in wavelength-integrated routines
grid: added routine calc_Rosseland_mean_opacity
main: added step which calculates Rosseland mean opacity in each GridVoxel
GridVoxel::calc_Rosseland_mean_opacity(): changed kappa to chi
EOS: now reads model atom data only once and sets up pointers in Atom::set_pointers()
Merge branch 'read_atomic_data_only_once' into develop