[divideTwoDatetimeDiffs] bugfixes in src/libmtime.f90, include/mtime_timedelta.h and src/mtime_timedelta.c

Fixes comparison of duration strings with more than 59 seconds, timedeltaToString and according tests.

Fix: mtime - Bug #5037. Type int too small in recently introduced numberOf400YearPeriods. Changing to int64_t

years.

Changing while to do-while to save un-necessary initialisation as suggested by Moritz. Ref commit c22f7403

Bug #4818: gettimedeltafromdatetime this routine should be in mtime_timedelta.c and not mtime_datetime.c

Adding the division routine to the Fortran interface and added testcases for the two additional routines from Florian.

Fortran interface updates: Events with DS now supports offset. isCurrent's allowed slack can be NULL