Fix issues with writing Exodus files after deleting nodes.
Fix issues with writing Exodus files after deleting nodes.

* mesh.n_nodes() does not return a reliable result when Nodes have been
  deleted but mesh.contract() has not yet been called.
* Must use node_num_map (or it's std::map equivalent,
  "libmesh_node_num_to_exodus") to write element connectivity arrays
  when there are holes in the node numbering.

Closes #753.

coreymbryant/libmesh,benkirk/libmesh,aeslaughter/libmesh,benkirk/libmesh,dknez/libmesh,capitalaslash/libmesh,vikramvgarg/libmesh,pbauman/libmesh,pbauman/libmesh,friedmud/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,balborian/libmesh,90jrong/libmesh,dschwen/libmesh,benkirk/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,vikramvgarg/libmesh,friedmud/libmesh,dschwen/libmesh,dschwen/libmesh,friedmud/libmesh,vikramvgarg/libmesh,svallaghe/libmesh,jwpeterson/libmesh,pbauman/libmesh,dknez/libmesh,libMesh/libmesh,aeslaughter/libmesh,dschwen/libmesh,BalticPinguin/libmesh,libMesh/libmesh,coreymbryant/libmesh,pbauman/libmesh,roystgnr/libmesh,vikramvgarg/libmesh,pbauman/libmesh,friedmud/libmesh,balborian/libmesh,benkirk/libmesh,benkirk/libmesh,roystgnr/libmesh,capitalaslash/libmesh,hrittich/libmesh,jwpeterson/libmesh,svallaghe/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,90jrong/libmesh,capitalaslash/libmesh,hrittich/libmesh,roystgnr/libmesh,capitalaslash/libmesh,dknez/libmesh,BalticPinguin/libmesh,libMesh/libmesh,hrittich/libmesh,friedmud/libmesh,coreymbryant/libmesh,90jrong/libmesh,90jrong/libmesh,svallaghe/libmesh,BalticPinguin/libmesh,vikramvgarg/libmesh,balborian/libmesh,roystgnr/libmesh,coreymbryant/libmesh,libMesh/libmesh,aeslaughter/libmesh,benkirk/libmesh,svallaghe/libmesh,giorgiobornia/libmesh,coreymbryant/libmesh,90jrong/libmesh,balborian/libmesh,balborian/libmesh,balborian/libmesh,giorgiobornia/libmesh,hrittich/libmesh,dknez/libmesh,aeslaughter/libmesh,hrittich/libmesh,giorgiobornia/libmesh,giorgiobornia/libmesh,svallaghe/libmesh,hrittich/libmesh,svallaghe/libmesh,coreymbryant/libmesh,benkirk/libmesh,90jrong/libmesh,giorgiobornia/libmesh,pbauman/libmesh,capitalaslash/libmesh,hrittich/libmesh,libMesh/libmesh,90jrong/libmesh,vikramvgarg/libmesh,capitalaslash/libmesh,svallaghe/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,coreymbryant/libmesh,dschwen/libmesh,benkirk/libmesh,aeslaughter/libmesh,dknez/libmesh,dschwen/libmesh,svallaghe/libmesh,pbauman/libmesh,capitalaslash/libmesh,hrittich/libmesh,aeslaughter/libmesh,friedmud/libmesh,friedmud/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,coreymbryant/libmesh,giorgiobornia/libmesh,dknez/libmesh,roystgnr/libmesh,balborian/libmesh,hrittich/libmesh,90jrong/libmesh,balborian/libmesh,libMesh/libmesh,balborian/libmesh,friedmud/libmesh,dknez/libmesh,libMesh/libmesh,jwpeterson/libmesh,friedmud/libmesh,vikramvgarg/libmesh,jwpeterson/libmesh,aeslaughter/libmesh,dschwen/libmesh,vikramvgarg/libmesh,pbauman/libmesh,dknez/libmesh,roystgnr/libmesh,benkirk/libmesh,BalticPinguin/libmesh,libMesh/libmesh,dschwen/libmesh,giorgiobornia/libmesh,pbauman/libmesh,giorgiobornia/libmesh,balborian/libmesh,aeslaughter/libmesh,aeslaughter/libmesh,svallaghe/libmesh,vikramvgarg/libmesh,90jrong/libmesh