Don't use PrintAs

It turns variables into strings, which may cause trouble elsewhere.
More robust handling of scalars
Don't allow indices to be used as tensor names
Tidy up some info messages
Fix error in cases where there is a partial derivative of a scalar
Allow partial derivative components to be given as positive integers

They are still assumed to be covariant components
It's no longer necessary to register numerical derivative operators with xTensor
Register numerical parameters with xTensor
Make xTensor aware of StepFunction
Allow single-character names to be used for tensors

If that character is already defined as an index, first undefine it
then define it as a tensor.
Update Burgers example to work with xTensor
Regenerate Burgers example with master branch commit eca6928f0542a28dcfad3dbc2bb3fec69a128cd6
Restore Advect example to almost how it was before the xTensor changes
Regenerate Advect example
Regenerate Laplace example with master branch commit eca6928f0542a28dcfad3dbc2bb3fec69a128cd6
Fix bug in registering parameters with xTensor
Support second (and higher) derivatives
Allow derivatives to work even if the symbol they're acting on hasn't been defined as a tensor
Update Laplace example to work with xTensor
Regenerate SimpleWave example with master branch commit eca6928f0542a28dcfad3dbc2bb3fec69a128cd6