Removed relative imports and plotting update

[abatten]

• Changes frb.py to _frb.py to remove the frb namespace. This will prevent users from accidently trying to use frb instead of Frb when defining the Frb object.
• Removed relative imports from not init files and made explicit imports from fruitbat.
• Changed the plot functions to have passed_ax and returns the figures instead of autosaving the file. Now only saves when given a filename.