absolute magnitude lookup for main sequence stars using estimateAbsoluteMagnitude

Looks up the value based on spectral type from a table. Interpolates gaps.
added estimate distance and estimate abs mag to star class
improved estimation of absolute magnitude with broad spectral types (‘G’) and invalid types
estimated distance flag
star distance is auto estimated when called if absent
added missing magnitude lookup file
fixed issue where estimate distance would return float(‘nan’) which isn’t checked with np.nan meaning nan * pq.pc is returned
additional value checking for estimate distance
fixed issue where distance was always calculated for stars, added tests
mag K to V conversion and table to enable other conversions
if magV is absent, converts from K