Adding support for Jungfrau .dat files (#152)

closes #150 

**Not addressed in this PR:** 

- pixels_per_frame, bytes_per_frame and tell should be made cost in
FileInterface

python/tests/conftest.py

@@ -0,0 +1,29 @@
+import os
+from pathlib import Path
+import pytest
+
+
+
+def pytest_addoption(parser):
+    parser.addoption(
+        "--files", action="store_true", default=False, help="run slow tests"
+    )
+
+
+def pytest_configure(config):
+    config.addinivalue_line("markers", "files: mark test as needing image files to run")
+
+
+def pytest_collection_modifyitems(config, items):
+    if config.getoption("--files"):
+        return
+    skip = pytest.mark.skip(reason="need --files option to run")
+    for item in items:
+        if "files" in item.keywords:
+            item.add_marker(skip)
+
+
+@pytest.fixture
+def test_data_path():
+    return Path(os.environ["AARE_TEST_DATA"])
+