Simplify implementation of toBasis

We don't need to use the full power of xTensor's ToBasis since
we're not really changing basis at all.
Improve DefineTensor

It actually works as it should now. It takes two arguments, the symbol
which will be used in tensor expressions and the symbol which represents
its numerical discretisation.
Update EM example to use DefineTensor
Fix the calculation of reflection symmetries
Regenerate EM example
Remove EM-xTensor example

The standard EM example now works with xTensor.
Improve error message
Make sure derivatives are expanded out fully so that ToCanonical can work on them
Add some InfoMessages to ExpandComponents
Make CreateGroupFromTensor also work on xTensor scalars.
Get rid of toBasis in favor of a simpler approach

We now define tensors and derivatives with indices which are
automatically converted to basis indices.
Add definitions for Euclidean tensor with indices of all character
Add support for defining scalars for use with xTensor
Improve some InfoMessage's
Use PrintAs[t] for the name of a tensor when converting it to Kranc form
Regenerate Advect example with master branch commit eca6928f0542a28dcfad3dbc2bb3fec69a128cd6
Update Advect example to work with xTensor
Regenerate Advect example