add doc/open_questions.md

Change-Id: Ied073d133a5f068e3de20a8d92e36dffbc4ebf08

doc/open_questions.md

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+
+  * Datatype for a mapping (like the calibration for the amagnet)
+  * How to set a Mapping or an enum (with all the name-value mappings) in the configfile?
+  * use toml vs. ini parser?
+
+  * error handling should be more consistent
+  * error-info stuff should be consistent
+
+  * dynamic device creation / modification (i.e. how to set the mapping of an enum during runtime?)
+
+  * propagation of units from main value to params
+
+  * switch params+cmds to 'accessibles'/attributes and keep in orderdDict?
+    -> needs Datatype for a function
+  * datatype for funcion: is argin a list anyway, or just one (structured) argument?
+    (so far we use a list and argout is a single datatype (which may be stuctured))
+
+  * polling: vendor specific or do we propose a common way to handle it?
+    (playground uses a per-device pollinterval. params may be polled less often)
+
+  * interface classes !!! (maybe with feature mixins like WindowTimeout?)
+
+  * hardware access: unify? how?
+
+  * demo-hardware?
+