Add test data and accessor function

MANIFEST.in

 include README.rst
 include LICENSE
 include requirements.txt
+include requirements-dev.txt
+
+recursive-include km3net_testdata/data

km3net_testdata/__init__.py

+from __future__ import absolute_import
+
 import os
 from pkg_resources import get_distribution, DistributionNotFound
 
-version = get_distribution(__name__).version
+import importlib_resources
+
+try:
+    version = get_distribution(__name__).version
+except DistributionNotFound:
+    version = "unknown version"
+__version__ = version
+
+
+try:
+    from importlib_resources import as_file
+except ImportError:
+    from importlib_resources.trees import as_file
+
+try:
+    from contextlib import ExitStack
+except ImportError:
+    from contextlib2 import ExitStack
+
+import atexit
+
+
+def data_path(filename, raise_missing=True):
+    """Return the absolute filepath for a given filename in test data"""
+    ref = importlib_resources.files("km3net_testdata.data") / filename
+    file_manager = ExitStack()
+    atexit.register(file_manager.close)
+    file_path = file_manager.enter_context(as_file(ref))
+    if raise_missing and not file_path.exists():
+        raise RuntimeError("Unknown or missing file: {0}".format(filename))
+    return str(file_path)
 
 
-def data_path(*path_elements):
-    return os.path.join(*path_elements)
+__all__ = ["data_path"]

requirements.txt

+importlib_resources
 setuptools_scm