trying to fix body forcing.

Also starting to add support for 2nd order (27-node) hexahedra in gmsh.
debugging and improving body force calculation.

Writing out a file for the high-order workshop test case containing
history of mass flux through inlet.
corrections to file write.
another write correction.
Fixing body force calc on GPUs.
Also commenting out 2 lines in shock capture kernel that were causing compile errors.
fixing file write after restart, so that old file is not overwritten.
Merge branch 'feature_fixbodyforce' of https://github.com/fpalacios/HiFiLES into feature_fixbodyforce
changing body forcing to maintain constant mass flux.
making the mass flux calculator converge.
fixing body force bug

The mass flux from previous timestep was not initialised correctly.
moving calculation of body force to start of timestep, not end. This appears to improve convergence behavior.
also unifying 'calculate' and 'evaluate' subroutines to simplify code.
some additional arguments have to be passed to CalcResidual to make this work.
attempting a fix to GPU body force.

Also cleaning up interface search.
further body force fixes

Adding body force to src_upts instead of viscous flux.
Some tidying up and renaming in eles.cpp.
trying out body forcing only every n steps
undoing previous commit - it didn't work.
fixing initial condition turbulent perturbation
Merge branch 'master' of https://github.com/fpalacios/HiFiLES into feature_fixbodyforce