Add docu for environment variables

docs/background.rst

@@ -4,6 +4,16 @@ Background information and general remark
 
 Cassandra_ is the current (2018) archiving solution used at KARA. There is a `JavaScript-Object-Notation`_-API available, that can be accessed from within the IBPT-LAN (CN, VPN or las-bernhard.anka.kit.edu).
 
+Usage with different hosts or ports
+-----------------------------------
+
+In case you want to run this scripts or library from a place where the host or port does not fit the presets, you need to provide them as environment variables.
+For example if you want to run your script ``cassandra_download.py`` and you forwarded ``ankacomm`` via SSH to the port 9999,
+you can then call your script in bash like::
+
+   CASSANDRA_HOST=localhost; CASSANDRA_PORT=9999 python3 cassandra_download.py
+
+
 Terminology
 -----------
 

docs/examples.rst

@@ -136,3 +136,16 @@ current directory.
 
 .. _pandas: https://pandas.pydata.org
 .. _`offset-aliases`: http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases
+
+Environment variables
+~~~~~~~~~~~~~~~~~~~~~
+
+Let us assume you tunnelled to a host in the machine network and now you do not have to use the archive gateway, but localhost as the host.
+This can easily be accomplished by setting the environment variable ``CASSANDRA_HOST`` and then running your script.
+
+code::
+
+    $ CASSANDRA_HOST=localhost python script.py
+
+.. note::
+    Unfortunately it is not sufficient to set the environment variable from inside your script or e. g. Jupyter notebook.

docs/index.rst

@@ -16,6 +16,7 @@ If I write Cassandra I am speaking of this Python class, if I write cassanda I a
    Examples <examples>
    (Command line) Scripts <scripts>
    Module Reference <api/cassandra>
+   Error messages and trouble shooting <troubleshooting>
    Changelog <changelog>
    License <license>
    Authors <authors>

docs/troubleshooting.rst

+.. _troubleshooting:
+
+Error messages
+==============
+
+URLError
+~~~~~~~~
+
+Typical causes for the URLError with the message `Maybe you're not inside the ANKA-LAN` are
+
+* You are not inside the IBPT office network nor in it via VPN. You should check this by getting the following URL.
+  http://ankasr-archiver.anka.kit.edu:9812/archive-access/api/1.0/archive/1/samples/A:SR:BeamInfo:01:Energy?start=1479813320000000000&end=1479813420000000000 Try to do it with wget or curl, because maybe you are working on a different computer than you think ;)
+* One of your provided ``PV`` names is wrong
+* Your start time is later that your end time
+* The cassandra archive is not responding correctly. Did you really double checked the other causes? Then get in contact with Sebastian Marsching or write a JIRA ticket.

src/cassandra/cassandra.py

@@ -16,6 +16,11 @@
     And finally the :class:`Pvs` class simply provides a :obj:`dict` of some
     EPICS Process Variables I often used or could not remember.
 
+    Possible environment variables are
+
+    * CASSANDRA_HOST
+    * CASSANDRA_PORT
+
 """
 from __future__ import with_statement