Fix examples

This contains the following:
 - Some examples are updated and improved, and are now dimensionally correct. In particular twophase.equelle.
 - Removed all extra heat equation examples as unneeded.
 - Added some missing comparison operators to the serial back-end (necessary for compiling changed examples).
 - Modified compile_examples.sh to use --nondimensional option only for an explicit list of not-yet-fixed examples.