enhance documentation

- flatten hierarchy (some links do not work when using folders)
- add a tutorial for programming a simple driver
- clean description using inspect.cleandoc
+ fix a bug with 'unit' pseudo property in a Parameter used as override

Change-Id: I31ddba5d516d1ee5e785e28fbd79fca44ed23f5e
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/25000
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

doc/source/secop_psi.rst

@@ -0,0 +1,27 @@
+PSI (SINQ)
+----------
+
+CCU4 tutorial example
+.....................
+
+.. automodule:: secop_psi.ccu4
+    :show-inheritance:
+    :members:
+
+
+PPMS
+....
+
+.. automodule:: secop_psi.ppms
+    :show-inheritance:
+    :members:
+
+LakeShore 370
+.............
+
+Calibrated sensors and control loop not yet supported.
+
+.. automodule:: secop_psi.ls370res
+    :show-inheritance:
+    :members:
+