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>

secop/core.py

@@ -32,6 +32,7 @@ from secop.lib.enum import Enum
 from secop.modules import Module, Readable, Writable, Drivable, Communicator, Attached
 from secop.properties import Property
 from secop.params import Parameter, Command, Override, usercommand
+from secop.poller import AUTO, REGULAR, SLOW, DYNAMIC
 from secop.metaclass import Done
 from secop.iohandler import IOHandler, IOHandlerBase
 from secop.stringio import StringIO, HasIodev