Always initalise h_celltype_field and h_cellid_field. This may not happen if cellTypeMonitorPlugin
has been loaded previously, by for example a different PDE Solver. If they're unset a
segfault is encountered prepCellTypeField
Fix typo of ArithmeticAverage in Potts3D
Merge pull request #6 from adrs0049/fixes

Always initalise h_celltype_field and h_cellid_field. This may not happe...