[!371] [bugfix] UGGRid loadbalance with datahandle

Merge branch 'fix/issue105-UGGrid-loadBalance-with-targetProcessors-and-handler-fails-for-element-data' into 'master'

ref:core/dune-grid> loadBalance with targetProcessors and handler for element data. See #102

The loadBalance method failed when no node data were given, element data were ignored.

Fixes #102

See merge request core/dune-grid!371

Closes #102 (closed)

(cherry picked from commit 880571a3)

2f45d010 [bugfix] UGGRid loadbalance with datahandle