>
<br><br>
 The temporal shape of the field is given as an expression (e.g., "cos(2.0*t)". The
 letter <i>t</i> means time, obviously. The expression is used to construct the function <i>f</i>
 that defines the field:
<br><br>

<br/><i>Options</i>:
<ul>
<li><b>tdf_cw</b>:  Explained above.
</li>
<li><b>tdf_gaussian</b>:  Explained above.
</li>
<li><b>tdf_cosinoidal</b>:  Explained above.
</li>
<li><b>tdf_trapezoidal</b>:  Explained above.
</li>
<li><b>tdf_from_file</b>:  Explained above.
</li>
<li><b>tdf_fr