Implements BwdTrans StdMat operator
This MR extends the initial prototype in the following ways:
- Link to Nektar++ to allow utilisation of existing code
- Add support for SIMD operations
- Add BwdTrans StdMat operator implementation
- Redefine BlockAttributes data structure
- Extend Field to support interleaving of data
The test code is also updated to use a Nektar++ XML file as input, rather than hard-coded elements.