Add file series reading and top level spatial partitioning

Now when you give it many filenames it will either make up a temporal collection
or a spatial collection out of them depending on the setting of FileSeriesAsTime.
The spatial collection is partitioned at the top of the tree rather than
at the bottom, because this significantly reduces XDMF's memory consumption
and speeds up the load and run times.

Change-Id: Ic218624ac515272a150de6201d0f4f0d61cdc716
fill in time varying graphs

now that libxdmf has it, fill in flagged code
also minor style updates, get rid of old/experimental comments

Change-Id: If7636ac4373b13bcd49bb231b40c9dc6591283a9
STYLE: pull helpers out and format all for vtkstyle

Change-Id: Iacbbebc12ae2456d250d37d28eab937c81f5fb5d
move strict partition multipiece constuction out to reader level

doing so lets me flatten the structure, which simplifies the code
because I can do it all at once and makes it easy to create the
multipiece dataset in the correct parallel invariant way.

also style changes and better comments

Change-Id: I99bad8672c0058842df618426489316567215b62
Merge topic 'newest-xdmf' into master

56e9a947 move strict partition multipiece constuction out to reader level
d8bf0337 STYLE: pull helpers out and format all for vtkstyle
f0853388 fill in time varying graphs
1e6c66b7 Add file series reading and top level spatial partitioning