This is an output array of length 3.  It contains the total force felt by atoms 
in <tt>group = 0</tt> and <tt>group = 4</tt>.

<p>
When the simulation is set up such that an indentor consisting of
these two groups of atoms is pressing against a substrate (consisting
of other groups of atoms), then <tt>tipforce</tt> measures the total
force on the indentor tip from the substrate.

