Add tests to check if OT files exist when NPROC > 1

[Martijn van den Ende]

From output.f90::ot_write:

open(pb%ot%unit,access='APPEND',status='old',iostat=ios)
if (ios>0) stop 'Fatal error: ot_write: Error opening a fort.18 file'
!JPA add test for the first time we try to open this file but it does not exist yet
!JPA add test to prevent appending data to a file from a previous simulation

These checks should be implemented in output.f90::ot_init; it is not necessary to check every output time step.

[Martijn van den Ende]

Addressed and fixed in #45