Orbit-predictor

* Fix version number

* Add new `SmartLocationPredictor` which uses SciPy to achieve
better performance and accuracy
* Allow change of tolerance for pass finding
* Fix corner case in eclipse computation

* Require sgp4>=2.3
* Use mean Earth radius for illumination computation
* Use current date if not given in `get_only_position` too
* Document `mean_motion` (finally!)
* Add `R_E_MEAN_KM`, `ALPHA_UMB` and `ALPHA_PEN` constants
* Add `period` property to predictors
* Add `get_shadow`, `get_eclipse_duration`, `get_beta`, `get_normal_vector`
and `eclipses_since` methods to predictors
* Make `sate_id` mandatory and `source` optional

* Add `osculating_elements` property to Position that computes
osculating orbital elements
* Add `is_sun_synchronous` function that checks if a Predictor
represents a Sun-synchronous orbit
* Add Earth flattening and light speed to constants

* Add get_shadow method that computes if a satellite is in shadow
* Add get_sun method that retrieves the ECI position of the Sun
* Add elevation and groundtrack generic APIs
* Rename LocationPredictor.propagator to predictor for clarity
* Make HighAccuracyTLEPropagator.propagator private
* Document Position.position_llh
* Add version to __init__.py
* Make propagate_eci public
* Raise error if propagation failed
* Fix J2Predictor.from_tle for non-epoch dates
* Add method to create predictor with repeating groundtrack
* Support Python 3.8
* Remove mentions to deprecated "precise" predictor

* Fix a bug that prevented predictors from being pickeable