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 Input(s):
 nodes  - number of nodes (total in mesh)
 nD     - number of dimensions
 g      - gravity
 mpData - material point structured array. Function requires:
           mpM   : material point mass
           nIN   : nodes linked to the material point
           Svp   : basis functions for the material point
           fp    : point forces at material points
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 Ouput(s);
 fext   - external fo