New readBal function

[varunagrawal]

This PR adds a new function readBal which returns the SfmData object, allowing for single line calling.

This function is particularly useful with the wrapper.

E.g.

SfmData data = readBal("example.bal")

rather than

SfmData data;
readBAL("example.bal", data);

Note I followed Google style when naming the function.

[varunagrawal]

@dellaert @ProfFan this is a quick one. :)