Fix various bugs

1. For printing, the the LaTeX printer and pretty printer can be used
for interactive sessions by subclassing the MBExpr class from the
sympy.Basic class

2. Minor bug fix for MBExprLatexPrinter and MBExprPrettyPrinter.

3. The trace_over_expr function has been changed to use the unevalated
Sum and Integral from summation and integrate, to maintain consistency
with other methods.