Integral representation support

Added support for the Galois module structure of units in Galois number fields.  This includes showing indecomposable modules on the corresponding Galois group page.

Also some minor improvements:
 * centered elements in the Frobenius table of a number field
 * made sure the polynomial of a number field has the +/- signs are in math mode (the situation was complicated by the fact that we want to allow line breaks in the middle of a long polynomial, so we are turning math mode on and off)