Merge branch 'GSOC-Integrator' into md_PRD
Cleanup  of config and solar radiation code.
- rename main config variable
- move solar radiation pressure code to general config processing function (calc_pInfo())
- add prdconst to config struct
Small fix, more cleanup:
- initialize j in the correct place
- better zero start date implementation
- more organized printf of file format
Restructure calculation of solar radiation pressure and beta
- beta based on solar luminosity value that can be changed
- beta in config_data
- calculate PRD only for mass > 0.
- more precise output for radius, beta and GM of the sun
- add unit comments to struct definition
Reimplementation of solar radiation pressure and Poynting-Robertson drag
- PRD corrected: absolute force was inaccurate
- Solar radiation pressure as a separate calculation, not modifying GM[sun]
- consistent commenting of units for most variables representing physical values
Add option to end on a precise time; Bugfix
- ENDONTIME shortens the last integration step to exactly meet the dictated final time
- Fix: calc_pInfo return value is int, not SpiceDouble
Merge branch 'md_PRD' into GSOC-Integrator