The function create an array containing the values, and also generate
the range in the last case.
Parameters :
rstring : str
A string on the form:
<value>, or <value>, <value>, <value>, ..., <value>, or <number>:<number>:<number>
dtype : special, optional
Datatype used for the numpy array. Default: str.
Returns :
retval : numpy.array
The method returns a numpy array contining the values in the string
either the single value, the list of values, or the range from first
(inclusive) to last (exclusive) by the step step.