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.

requested review from @dmoxey

assigned to @ccantwel

mentioned in commit 3777665e

merged