Suppress warnings in LCG DEV(3/4) slots
The newer version of cppgsl in the dev3/dev4 nightly slots (3.x) changes the type of span::size() from a signed to unsigned int type. This causes a number of comparison and narrowing warnings to occur, due to slightly sloppy usage of these types. e.g.
This MR addresses them in a way that works with both the old and new cppgsl versions.