Previously, vtkAVSucdReader for ASCII input was based on a simplifying
assumption that all nodes and cells are numbered consecutively with node
IDs starting at 0 or 1, and cell IDs starting at 0. When this assumption
did not hold, the reader produced incorrect output or could cause
a segfault.

However, according to:
http://help.avs.com/Express/doc/help/reference/dvmac/UCD_Form.htm
any numbering is allowed in UCD ASCII files:
"... Node ids must be integers, but any number including non sequential
numbers can be used. ..."

This commit fixes the issue by introducing a simple mapping from node
and cell IDs read from the file to those used by VTK.