This class was used to store a group of from field data in a topology
map. However, the fetching has been changed to use a customized class
for each type of fetch that can be optimized for the fetch type and does
not require to know the number of items in the fetch at compile time.
Thus, this class is no longer needed, so it is being removed.
This change removes the requirement to specify some maximum cell length
in each of the worklets, which is basically impossible. It also makes
some of the loading more lazy, which might help reduce the number of
registers required in a worklet.
In the CellSet and related classes, a connection was referred to by a
"from" topology element and a "to" topology element. However, in the
worklet control signature tags the elements were referred to by "src"
and "dest." To make things consistent, use "from" and "to" everywhere.
Most of VTK-m follows the convention of calling the 0D topology elements
"points" (which follows the convention of VTK). However, there were
several places where they were referred to as "nodes." Make things
consistent by calling them points everywhere.
Also merged some redundant ExecutionSignature tags.
