add [h5,nc]ndims. [h5,nc]size scalar is empty. h5save write scalar size accurately

• add functions "h5ndims" and "ncndims" to get rank of disk variables
• "h5size" and "ncsize" return empty [] if disk variable is scalar. This is like Python etc.
• "h5save" better respects optional "size" argument to optionally write scalars instead as 1 or more dimensions e.g. [1,1,1]. This is necessary to interface with other languages.