Skip empty lines when comparing upscaling results.

The 'line' obtained from std::getline() will be empty if the input
line is blank.  Treat such lines as comments.

Incidentally, this also fixes the tests

   - compare_upscale_elasticity_mpc_EightCells
   - compare_upscale_elasticity_mortar_EightCells

when using "libstdc++"'s "debug" mode (-D_GLIBCXX_DEBUG=1
-D_GLIBCXX_DEBUG_PEDANTIC=1).
Merge pull request #78 from bska/fix-nullptr-deref

Skip empty lines when comparing upscaling results.