add point source with line piston far-field directivity

[fietew]

[mgeier]

Same procedure as #156:

$ python3 -m pycodestyle sfs/fd/source.py

Same Sphinx warning:

sfs/fd/source.py:docstring of sfs.fd.source.point_line_piston:19: WARNING: Explicit markup ends without a blank line; unexpected unindent.