Make numDimension, numNodesPerEle, numGaussPoints attributes of FormBackend

[kynan]

This requires changing the way DimIndex, BasisIndex and GaussIndex work:

• extent() is now a method of the CodeIndex base class and returns the _extent attribute
• FormBackend has 3 methods buildDimIndex, buildBasisIndex, buildGaussIndex which return a corresponding index of the extent set from the corresponding attribute of FormBackend

ElementIndex (specific to the CUDA backend) does not derive from CodeIndex anymore.

ExpressionBuilder and QuadratureExpressionBuilder now need to hold a handle to the FormBackend so they can call the build*Index methods. For the same reason this is also true for the Op2KernelParameterGenerator.

All induction variable methods have been removed and replaced by a call to the name() method of the corresponding Index.

ExpressionBuilder and QuadratureExpressionBuilder classes have been moved to expression.py.

[gmarkall]

Looks good. Just a couple of quick questions:

1. Why does ElementIndex no longer derive from CodeIndex?
2. There's some very long lines now, as a result of DimIndex(0) -> self._form.buildDimIndex(0) etc... It might be nicer if the longest ones were a little shorter (e.g. so they can be read without scrolling the github web interface would be enough).

[kynan]

1. CodeIndex now has a member _extent which is returned by the extent() method as a Literal. ElementIndex.extent() however returns a Variable, so I felt there wasn't too much in common anymore.
2. Forgot about that, will fix