gcc 8.3 compilation warnings

[pinkenburg]

Three sources give warnings when compiled with gcc 8.3 and std=c++17:

src/simulator.cpp:769:90: warning: narrowing conversion of 'row' from 'unsigned int' to 'int' inside { } [-Wnarrowing]
StTpcLocalSectorCoordinate xyzW{at_readout.x, at_readout.y, at_readout.z, sector, row};

test/test_tpcrs.cpp:45:84: warning: no return statement in function returning non-void [-Wreturn-type]
Diff& operator +=(const Diff& b) { total += b.total; unmatched += b.unmatched; }

include/tpcrs/detail/digitizer.h:145:1: warning: no return statement in function returning non-void [-Wreturn-type]

[pinkenburg]

simulator.cpp warning is fixed in my PR #16