Add deprecation warning when using opmd_viewer, and update CHANGELOG

opmd_viewer/__init__.py

@@ -0,0 +1,29 @@
+"""
+This is a stub that detects whether the user is attempting to use the
+old import statement from openPMD-viewer 0.X (`import opmd_viewer`).
+
+In that case, an exception is raised that prompts the user to use the new API.
+"""
+# Define the version number
+from openpmd_viewer import __version__
+__all__ = ['__version__']
+
+raise DeprecationWarning("""
+It looks like you are trying to use the API from openPMD-viewer version 0.X
+but the installed version on your system is openPMD-viewer version 1.X.
+
+* If you wish to use the new openPMD-viewer version 1.X, the import statement
+should use `openpmd_viewer` instead of `opmd_viewer`, e.g.:
+```
+from openpmd_viewer import OpenPMDTimeSeries
+```
+Please have a look at the list of the changes introduced in version 1.X here:
+https://github.com/openPMD/openPMD-viewer/blob/upcoming-1.0/CHANGELOG.md#10
+In particular, note that `get_particle` now returns particle positions in
+meters (not in microns anymore).
+
+* If you wish to go back to the old openPMD-viewer version 0.X, use:
+```
+pip install openPMD-viewer==0.9
+```
+""")