Make standalone equation-filtering command

[Chris Warburton]

IsaCoSy includes constructors, even if they're not in the sample. This
will skew its precision/recall scores, so we should take this into
account: look through all of the resulting equations and, if they
contain any constants which don't appear in the sample, then either:

 - If the sample contains "constructor-" or "destructor-" names
   (possibly encoded), then see if these unknown names match those
   sampled names, modulo the "constructor-" or "destructor-" prefix. If
   so, replace these unknown names with the corresponding sampled ones.
 - Otherwise, drop that equation.

IsaCoSy output should be piped into this filter when benchmarking, but
it shouldn't be hard-coded into the isacosy command itself, since it
would be nice to run that command and see all of the equations.