Increased compiler compatibility
This MR introduces assorted minor changes (syntax fixes, compiler bug workarounds, code clarifications) that mainly enable SIESTA compilation with NAG 7.1 and CRAY 11, both serial and parallel, with and without OpenMP. AOCC 3 and Flang 13 are still affected by some bugs, see #149.
Each commit addresses and documents each of the issues independently.
Edited by José Mª Escartín