Support conversion to single precision

[mclarsen]

We should support this. VTKm might have evolved past the point where rectilinear grids must be float_default

[cyrush]

precision convert could be a transform filter?

[mclarsen]

It has been a while since I wrote this, but what I think I was getting at was not to be locked into types. In the past we had to make the coordinate systems of rectilinear grids whatever the FLOAT_DEFAULT was or it would fail. Now that vtkm supports virtual coordinate systems, this is not a problem any more and we took out the static asserts.

That said, I would be generally useful to be able to convert double to float for saving space.