thread_private class for static OpenMP objects acts as iterator

EndModification do not unnecessarily delete orphan elements

Fixed Face::getNodes behaviour inside BeginModification and
EndModification region (same fix as in andre_brench)

Added template access functions for AbstractEntry class, allows to get
(matrix of) values, unknown indices or unknowns themselves depending on
template parameter

Examples/OldDrawGrid:
* Adjustments for svg output of color gradient in color bar (previously
was not properly working in Inkscape)
* Preparations for isosurface extraction code

Thanks to Matvey Kraposhin

option for prismatic grid with same directions

not tested yet

For faster calculation of closest cluster, seems to be slower in
practice

Add algorithm to balance number of points among processors

Still have to add algorithm to balance number of points. each processor
works on

Testing algorithm before adding to partitioner

PrepareReceiveInner was not called for sparse tag

Possible fix in packtagdata/unpacktagdata - mismatch between amount of
calls to MPI_Pack and MPI_Unpack

GridTool for Kmeans clustering, should eventually move to Partitioner
class