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.

debian/control

@@ -13,7 +13,7 @@ Build-Depends:
  python3-all,
  python3-argparse-manpage,
  python3-igwn-auth-utils (>= 0.3.1),
- python3-ligo-segments,
+ python3-igwn-segments,
  python3-pytest (>= 2.8.0),
  python3-requests-mock,
  python3-setuptools,
@@ -26,7 +26,7 @@ Depends:
  ${misc:Depends},
  ${python3:Depends},
  python3-igwn-auth-utils (>= 0.3.1),
- python3-ligo-segments,
+ python3-igwn-segments,
 Description: GW data discovery Python 3 library
  The DataFind service allows users to query for the location of
  Gravitational-Wave Frame (GWF) files containing data from the