Draft: Support multiple APIs

This MR implements Support new gwdatafind server APIs (#28) by refactoring gwdatafind.api into a subpackage, and implementing a decorator for the gwdatafind.ui methods that selects the xxx_path function from the right API module dynamically.

Also adds a new -A/--api option for the command-line interface.

gwdatafind/api/__init__.py

@@ -15,10 +15,27 @@
 # You should have received a copy of the GNU General Public License
 # along with GWDataFind.  If not, see <http://www.gnu.org/licenses/>.
 
-"""API definitions for the GWDataFind Client."""
+"""API definitions for the GWDataFind Client.
+
+Each API module must define the same set of functions that
+return paths to be queried on the host to return the various
+endpoints.
+
+The required functions and signatures are:
+
+- ``ping_path()``
+- ``find_observatories_path()``
+- ``find_types_path(site=None)``
+- ``find_times_path(site, frametype, start, end)``
+- ``find_url_path(framefile)``
+- ``find_latest_path(site, frametype, urltype)``
+- ``find_urls_path(site, frametype, start, end, urltype=None, match=None)``
+"""
 
 # list of APIs
 APIS = (
     "ldr",
     "v1",
 )
+
+DEFAULT_API = "ldr"