This PR adds two helper methods to ISAM2Result which allows wrapper access to the errorBefore and errorAfter nonlinear errors of ISAM2.

The helper methods are setup to leverage the boost optional capabilities, such that if evaluateNonlinearError is False, they will return nan. This works great with the python wrapper i.e. the result is printed out as nan.

Example use case:

isam2_parameters = gtsam.ISAM2Params()
isam2_parameters.setEvaluateNonlinearError(True)
#
#
#
result = self.isam.update(graph, initial_estimate)
print("Before Error {0}".format(result.getErrorBefore()))
print("After Error  {0}".format(result.getErrorAfter()))

This method has proven super useful in helping to debug various setup issues in ISAM2 by evaluating the error.

Any feedback involving location of tests for these methods?