g> = <span style="color:red">trexp</span>(<strong>sigma</strong>) is the matrix exponential (4x4) of the se(3) element <strong>sigma</strong> that
yields a homogeneous transformation  matrix (4x4).

</p>
<p>
<strong>T</strong> = <span style="color:red">trexp</span>(<strong>tw</strong>) as above, but the se(3) value is expressed as a twist vector <strong>tw</strong>
(1x6).

</p>
<p>
<strong>T</strong> = <span style="color:red">trexp</span>(<strong>sigma</strong>, <strong>theta</strong>) as above, but se(3) m