Moved sg_filter.py into a sg_filter package

wafo/sg_filter/__init__.py

@@ -0,0 +1,2 @@
+from wafo.sg_filter._core import *
+from wafo.sg_filter import demos

wafo/sg_filter/tests/conftest.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+"""
+    Dummy conftest.py for wafo.
+
+    If you don't know what this is for, just leave it empty.
+    Read more about conftest.py under:
+    https://pytest.org/latest/plugins.html
+"""
+from __future__ import print_function, absolute_import, division
+
+import pytest  # @UnusedImport