adding exact SGS 'model'.

This is a new kind of LES model in which the velocity is explicitly
filtered using an invertible filter (the inverse Helmholtz operator)
allowing the subgrid stresses to be exactly reconstructed from known
quantities. See Germano (1986) for details.

Also removing some unused dynamic LES options from schema.
fixing typo

Fixing a typo in dynamic LES which led to incorrect value of gamma
coefficient.
minor changes to dynamic LES

Neatening up the pointer and flag initialisations.
Ensuring that Cs does not become infinite when denominator is zero.
Minor change to leading whitespace.
Adding back in the choice of length_scale_type for filtering.
adding back in the choice of scalar or tensor filter width in filtering routines for dynamic LES
This is a minor bug fix for the build framework. The Make.dependencies is not in sync with the code and thus causes make makefiles
to fail.
Merge branch 'turbulence' of https://github.com/FluidityProject/fluidity into turbulence
Merge branch 'master' of https://github.com/FluidityProject/fluidity into turbulence
Merge branch 'master' of https://github.com/FluidityProject/fluidity
Merge branch 'master' into turbulence