Initial commit of vtu cell data export logic

This is a bit of a mess currently but it works; p4est_vtk_write_header
has been hijacked to append the custom cell data to the tree/level/rank
cell data exported therein. I do not like this solution but I do not
want to mess too much with p4est's paradigm without discussing it with
the main developers.
Minor spelling corrections
Merge remote-tracking branch 'upstream/develop' into develop
Merge branch 'develop' into vtk_cell_data
Completely re-factor custom cell data vtu export

This commit totally reworks the custom cell data export introduced
previously. The functionality is largely the same but the code is much
cleaner. Following is a summary of the changes:

* p{4,8}est_vtk_write_file and p{4,8}est_vtk_write_all have been
  reverted back to their previous functionality for backwards
  compatibility.  I.e. these functions do not support custom cell data.

* p{4,8}est_vtk_write_header now ONLY writes point and cell data and
  appropriate header text; all logic related to custom point/cell data
  export has been removed and is now self contained.

* Two (technically three) new functions have been introduced:
  p{4,8}est_vtk_write_cell_data and p{4,8}est_vtk_write_point_data.
  p{4,8}est_vtk_write_point_data is really just a wrapper to the
  function vp{4,8}est_vtk_write_point_data which facilitates passing
  variable argument lists from a calling function to the called
  function. These functions take a variable number of custom point/cell
  data and pass them one set at a time to the
  p{4,8}est_vtk_write_{point,cell}_{scalar,vector} functions that write
  them to the vtu file.

* No meta data is written to the .pvtu file in any of the
  p{4,8}est_vtk_write_{point,cell}_{scalar,vector} functions. The meta
  data associated with the custom point/cell data is now all written by
  p{4,8}est_vtk_write_{point,cell}_data.

* p{4,8}est_vtk_write_footer is largely unchanged except that the footer
  data associated with the custom point data has been moved into
  p{4,8}est_vtk_write_point_data.
Update the vtk documentation

This commit updates the p{4,8}est_vtk.h files with appropriate
documentation explaining the new custom cell data export functionality.
Improve error reporting

This commit generalizes the error reporting in p4est_vtk_write_footer to
be correct for both a p4est or a p8est.
Merge branch 'develop' into vtk
A couple tweaks to the vtk API
Merge branch 'vtk' into wrap