Removed pyslalib from required dependencies (now optional)

AstroAtmosphere/refractivityModels.py

@@ -193,7 +193,12 @@ def slalib(l, T=288.15, p=101325, H=0, h=0, lat=-24, tlr=0.0065, eps=1e-10):
 		Returns:
 		n - Refractive index of atmospheric air at the given conditions
 	'''
-	from pyslalib import slalib
+	try:
+	    from pyslalib import slalib
+	except ImportError as e:
+	    print('pySLALIB package not found! Try installing it first.\nReturning n=1.')
+	    return 1
+	
 	zObs = np.deg2rad(45.06563)
 	p = p/100
 	R = []