Tahoe Code SVN development

Fleshed out and corrected a lot of information for element with Dieterich
and Linker friction model on band. One-element problems seem to be running.

enhancement functions appear to be calculated correctly

further updates

adding start vector

modified gradient enhancement function calc

add criterion to determine normal that is consistent with current deformation

Adding new derived class enhanced strain element with non-constant
friction. Updated base classes to better handle inheritance, breaking out
some functions, etc.

Updated to correctly calculate tangent stiffness for case when residual
cohesion goes to zero. Sometimes (though rarely) the answer is simple.

Lots of wordk. Updated to actually check for yielding along shear
band. Made sure slip direction was dialtatant. Track residual cohesion and
do not allow this to go below 0. Tried to create a routine to track
stiffness on step where cohesion does go to zero, but this gives only
linear global convergence. Updated to handle reverse loading and guarantee
that jump is positive along slip direction.

maintenance commit

updates for dissipation factor calculation