Switch to Richardson iterations with the LU preconditioner for the CFBC verification test.

I hope this will take care of the nightly test failure.
Fix the "put temperature at depth" bootstrapping rule.
Fix pismmerge.

(pismmerge did not include global attributes when merging all variables.)
Add PISMTime::current_years() to ease debugging.

This method is disabled if PISM is compiled with PISM_DEBUG==0. This
is intentional.