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`).

This problem was discovered while testing PR #77