Generalise optional "unit" attribute for all datasets

Move the description of the "unit" attribute from the trajectory group
to the general section on time-dependent data groups.

draft.rst

@@ -111,8 +111,10 @@ As an example, here is the data for the position in a group of particles::
       \-- group1
            \-- position
                 \-- value
+                     +-- (unit)
                 \-- step
                 \-- time
+                     +-- (unit)
 
 where the first dimension of "value" must match the unique dimension of "step"
 and "time".
@@ -125,6 +127,11 @@ links to the "step" and "time" datasets of one data group. If data are sampled
 at different times (for instance, one needs the positions more frequently than
 the velocities), "step" and "time" are unique to each data group.
 
+The datasets "value" and "time" may possess an optional attribute "unit" that
+gives the physical unit of their respective data ("nm" for the position, for
+instance).
+
+
 Trajectory group
 ----------------
 
@@ -167,10 +174,6 @@ or C++ program may thus declare r\[N\]\[D\] for the coordinates array while the
 Fortran program will declare a r(D,N) array (appropriate index ordering for a
 N atoms D dimensions system) and the HDF5 file will be the same.
 
-The "position", "velocity" and "force" datasets possess an optional attribute
-"units" that gives the units of their respective data ("nm" for the position,
-for instance).
-
 The content of the trajectory group is the following::
 
     trajectory