Create new MPI communication routines for DG implementation

This MR replaces the existing Gs implementation of the DG parallel exchange routines with a custom class AssemblyCommDG. This works in a similar way to Gs, timing a number of different parallel implementation strategies at startup:

• AllToAll and AllToAllV perform two variants of all-to-all communication (without and with variable offsets, respectively). The latter is almost always preferable but in cases where communication is very similar (e.g. structured grids) the former may be preferable.
• Pairwise implements asynchronous send/recv calls, which seems to be generally the preferred implementation on current tests (and essentially replicates the Gs pairwise routines);
• Neighbour_Alltoallv uses an MPI 3.0 extension that allows for construction of a distributed graph representing the mesh topology, which seems to be the preferred exchange routine at low element to core counts.

The strategy adopted is to declare edges lying on partition boundaries to always be left-adjacent (or fwd edges). The AssemblyCommDG routines then take care of communicating the fwd space to the bwd space of the corresponding element on the remote processor. This has the advantage of removing the need for the normal negation routines, which have all been removed.

Timing test

1.6m quad cell ADRSolver case P = 8, run for 10s - measuring CPU Time every 0.1s and recording average of the 100 measures.

Number of Nodes (e.w. 20 cores):	1	2	4	8	16	32	48	64
Avg. Master Time (s):	58.5694	28.7446	13.2829	6.1681	2.9595	1.5473	0.9953	0.7530
Avg. New Branch Time (s):	58.4441	28.5092	12.9718	6.0861	2.8885	1.4673	0.9557	0.7230
Difference (%):	-0.21%	-0.82%	-2.34%	-1.33%	-2.40%	-5.17%	-3.99%	-3.98%