Switch to a simple static member function to implement singletons, which is guaranteed to be thread-safe in C++11.

Updated the branch to use input file specified as USERDEFINEDTYPE="Womersley:WomParams.xml" which takes the form:
<NEKTAR>
  <WOMERSLEYBC>
    <WOMPARAMS>
      <W PROPERTY="Radius" VALUE="0.5" />
      <W PROPERTY="Period" VALUE="1.0" />
      <W PROPERTY="axisnormal" VALUE="0.0,0.0,1.0" />
      <W PROPERTY="axispoint" VALUE="0.0,0.0,0.0" />
    </WOMPARAMS>

    <FOURIERCOEFFS>
      <F ID="0"> 0.600393641193,    0.0               </F>
      <F ID="1"> -0.277707172935,   0.0767582715413   </F>
      <F ID="2"> -0.0229953131146,  0.0760936232478   </F>
      <F ID="3"> 0.00858135174058,  0.017089888642    </F>
      <F ID="4"> 0.0140332527651,   0.0171575122496   </F>
      <F ID="5"> 0.0156970122129,   -0.00547357750345 </F>
      <F ID="6"> 0.00473626554238,  -0.00498786519876 </F>
      <F ID="7"> 0.00204434981523,  -0.00614566561937 </F>
      <F ID="8"> -0.000274697215201, 0.000153571881197 </F>
      <F ID="9"> -0.000148037910774, 2.68919619581e-05 </F>
    </FOURIERCOEFFS>
  </WOMERSLEYBC>
</NEKTAR>

Fixed issue with default values not being specified causing a problem with teh bfs_probe regression tst. Also added some documentation on using plt files output

Additional required package for compiling the documentation on very basic systems (e.g. Win10's linux subsystem)