ATC: fix_modify AtC output nodeset

# fix_modify AtC output nodeset

## syntax

fix_modify AtC output nodeset <nodeset_name> <operation>

- nodeset_name (string) = name of nodeset to be operated on
- operation (keyword/s):
  sum = creates nodal sum over nodes in specified nodeset

## examples

`fix_modify AtC output nodeset nset1 sum`

## description

Performs operation over the nodes belonging to specified nodeset and outputs resulting variable values to GLOBALS file.

## restrictions

## related

see fix atc command

## default

none