v0.5.0

Changes in structure of core classes:

• begin adding functions and methods to use mjd instead of jd
• Standardize Observer methods. Use method() to accept datetime objects, _method_mjd() to accept mjd time objects
• Change SGP4Predictor to SGP4Propagator
• Update satellite propagator methods to correspond to Observer methods
• Update tests and benchmarks with new class names and methods
• Remove orbit predictor Location class as inherited base class
• Create new Orbit class that can be initialized from a TLE.
• Refactor the SGP4Propagator to not call a source class directly. Instead initialize from Orbit or TLE object
• Create tests to ensure that SGP4 satrec propagator is created correctly from TLE object

Changes to pass iterators:

• Change Observer pass iterator algorithms to callable methods rather than subclassing Observer()
• Use MJD instead of JD in pass iterators. Update propagation and coordinate transformations to use MJD.
• Update tests to use new observer pass iterator methods
• move visual pass detail computation to separate function
• Performance boost: orbit_predictor pass iterator is now 1.2x faster

Misc:

• Remove old core.py functions. These can now be easily replicated with the current api
• Enforce mjd2datetime to always use microseconds.
• Rename satellite modules to sgp4.py and kepler.py