Merge branch 'release/0.1.7'
bug fixes to empty Planet class creating

Change name generation when empty to allow for the parent being empty
Doesnt try to generate params from a None object (empty creation)
tests empty class creation
Example system and star with unit tests, addition of units such as age and magnitudes to star class
planet object, added discovery tags and last update, reformatted transit time in days
added example system hierarchy
Merge branch 'feature/exampleSystem' into develop