Use enum on aravis driver

[callumforrester]

Fixes #411

[DiamondJoseph]

Pretty sure we should support LineN for N in {1,2,3,4}.
Docstring should be updated above AravisTriggerSource

[callumforrester]

Line1 was the only thing that came back in the error, I presume if we supported LineN, then the connection would break again. Need @coretl's input

[coretl]

Manta supports Line1 and Line2. Mako supports only Line1. We list the lowest common denominator for type checking, then it's typed as a string at runtime so we can do sig.set(f"Line{n}")