add test program to test attribute copy

[wkliao]

This program tests how to copy an attribute from a file to another. Note that
there is no ncmpi_copy_att() function in PnetCDF-Python. This is because
copying a python object is much simpler than in C. In C, without
ncmpi_copy_att(), one have to do the followings.

1. find the size and type of the attribute from the source file
2. malloc space, read from the source file
3. write to the destination file
4. free the buffer

In python, this can be done in two lines of codes:

att = source_file.get_att("history")
destition_file.put_att('history', att)