ake the boundaries of the simulation
 box absorbing.

<br/><i>Options</i>:
<ul>
<li><b>not_absorbing</b>:  No absorbing boundaries.
</li>
<li><b>sin2</b>:  A <math>\sin^2</math> imaginary potential is added at the boundaries.
</li>
<li><b>mask</b>:  A mask is applied to the wavefunctions at the boundaries.
</li>
<li><b>exact</b>:  NOT WORKING YET!
 An exactly absorbing scheme is used for open boundaries. This feature
 comes from transport calculation and assumes that on <tt>OpenBoundariesNLeads</tt>
 sides 