Change default mesh grading?

[parrangoiz]

Anecdotally, α_default = 0.9 in MeshingParameters sometimes results in gmsh crashes, especially during curved mesh optimization. Also anecdotally the value 0.75 yields much more stable results in my own experience. I'd suggest making the change 0.9 -> 0.75. @hughcars

[hughcars]

That's perfectly reasonable, and closer to the kind of grading we would want for p~3 simulations, so totally reasonable. We should probably also have a "global" refinement parameterization to allow for refining the size field everywhere. I think merge #112 then I can modify the size field formula and change the default all at the same time.