[Users] CarpetRegrid2 change

[Erik Schnetter]

I just committed a change "CarpetRegrid2: Try harder to combine
different refined regions". This may affect the grid structure you see
during time evolution.

When a refinement level consists of several regions, then
CarpetRegrid2 tries to combine these into a single, cuboid region,
adding additional refined points in the process. Having fewer regions
with a more regular shape helps efficiency. CarpetRegrid2 then checks
how many new refined points needed to be added in the process. If this
number is small, the single region is kept; if this number is too
large, the single region is discarded, and the original set of regions
is used instead. The parameter "CarpetRegrid2::min_fraction"
determines this, and setting min_fraction=1.0 disables this grid
structure optimisation.

This change slightly modifies this algorithm in CarpetRegrid2, and may
thus lead to a different grid structure.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/