Improve error messages for boundary conditions in 1D

[wdhawkins]

In 1D, it's not necessarily obvious that boundary conditions should be zmin and zmax. If the user supplies something else, say xmin and xmax, the code throws an error, but it is not helpful:

[1]  **** ERROR ****  IndexError: map::at:  key not found
[1]  **** ERROR ****
[1]  **** ERROR ****  At:
[1]  **** ERROR ****    TransientKEigenXSMapSwap.py(75): <module>

We need to improve our error messages for this case.