equation bank

[lheagy]

In order to upgrade to the latest version of the sphinx (closing issue #428), I removed the use of the equation bank in pr #546. The issue was that sphinx expects that each labeled equation have a unique label. However, when we were including equations from the equation bank, the same equation (and thus label) would appear on multiple pages. In order to both use the equation bank and have unique labels, we need to over-ride the include statement to do something along the lines of

.. include_equation:: ampere_maxwell.rst
    :label: ampere_maxwell_derivation 

so that each time the equation is input, the label is unique