Python 78a

This is the release for Python 78a.

This is intended as a stable verison, pre Nick's changes which incorporate the matrix ionziation scheme.

Only physics change is the one described by #90 - The two level atom code in lines.c now permits the model of mean intensity (power law or exponential) to be used when calculating radiative excitation. The others all fixed small bugs or small changes to improve clarity or stop errors.

Pull requests in this release:
#99 - Reporting issues from mean_intensity
#98 - Fix for infinite entries in spec files - default p_escape to 1.0 for low and high tau
#96 - Dynamic allocation of NION dimensioned arrays in plasma structure
#88 - New scheme for two level atom
#87 - definitions for ion and nebular modes

And the following small changes in commits:
agnwinds/python@ebc5922 - Put check around windsave command in python, so only rank0 writes
agnwinds/python@965d493 - Added pi_rates.c, not yet used
agnwinds/python@f64bb70 - corrected a load of set but unused variables which threw compiler warnings
agnwinds/python@2949341 - Makefile to uses -fno-aggressive-loop-optimizations for gcc > 4.8, bandaid fix for #100
agnwinds/python@2949341 - modified xdefine_phot so only calculate level emissivities if geo.pcycle ==0