vector.__repr__ method now follows convention

[aherrera1721]

This PR updates the vector.__repr__ method to follow the Python convention which states, "If at all possible, [the returned value] should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment)."

Now, calling eval(repr(v)) (where v is a vector) returns an equivalent vector.

[mwcraig]

Thanks @aherrera1721 for raising this issue and fixing it so quickly!