fix issue 59 | samples_from_md should only be used for MDs (#60)

* require infile.sim.hdf5 with velocities
* fail when number of requested samples is larger than number of samples

src/libolle/gottochblandat.f90

@@ -1542,6 +1542,8 @@ subroutine lo_stop_gracefully(msg,exitcode,filename,line)
         write(*,*) 'exit code 7: MPI error'
     case(8)
         write(*,*) 'exit code 8: Feature removed'
+    case(9)
+        write(*,*) 'exit code 9: Bad inputs.'
     end select
     write(*,*) ''
     do i=1,size(msg)

src/samples_from_md/main.f90

@@ -26,7 +26,17 @@ program samples_from_md
 ! Seed random numbers
 call tw%init(iseed=0, rseed=walltime())
 ! Get simulation
-call sim%read_from_file(verbosity=1)
+call sim%read_from_hdf5('infile.sim.hdf5', verbosity=1)
+
+! Stop when velocities are not present
+if (.not. allocated(sim%v)) then
+    call lo_stop_gracefully(['No velocities present in the simulation. Is this an MD?'], 9)
+end if
+
+! Check if the number of samples is not too large
+if (opts%n .gt. sim%nt) then
+    call lo_stop_gracefully(['Number of samples is larger than the number of timesteps in the simulation.'], 9)
+end if
 
 ! List of all the samples
 allocate (indices(sim%nt))