Add brackets arount logical statements.

src/Afterburner.cc

src/Arith.cc

src/Ensstat3.cc

src/Exprf.cc

src/Filedes.cc

src/Gradsdes.cc

src/Gridboxstat.cc

src/Intlevel.cc

src/Selbox.cc

src/Selvar.cc

src/Selyearidx.cc

src/Timselpctl.cc

src/Trend.cc

src/Ydaypctl.cc

src/grid_point_search.cc

src/process.cc