Move figure.py and base_plotting.py under a pygmt/figure folder

Also fixed some relative imports.

Author: weiji14

pygmt/figure.py renamed to pygmt/figure/__init__.py

@@ -10,10 +10,10 @@
 except ImportError:
     Image = None
 
-from .clib import Session
+from ..clib import Session
 from .base_plotting import BasePlotting
-from .exceptions import GMTError, GMTInvalidInput
-from .helpers import (
+from ..exceptions import GMTError, GMTInvalidInput
+from ..helpers import (
     build_arg_string,
     fmt_docstring,
     use_alias,

pygmt/base_plotting.py renamed to pygmt/figure/base_plotting.py

@@ -6,9 +6,9 @@
 import numpy as np
 import pandas as pd
 
-from .clib import Session
-from .exceptions import GMTError, GMTInvalidInput
-from .helpers import (
+from ..clib import Session
+from ..exceptions import GMTError, GMTInvalidInput
+from ..helpers import (
     build_arg_string,
     dummy_context,
     data_kind,