SubDevDiag: add documentation to class methods; add it to the docs, add tests

Yury Matveevrequested to merge
improve-subdevdiag into develop

Closes #707 (closed)

The class was already exposed, so there's nothing to do here. I just removed unnesessary anymoer overload of get_sub_devices (since now Tango::DevVarStringArray is automatically castes with our casters)

  1. I added SubDevDiag into the docs, in the Server/Util API section
  2. I also added a method description. It is copied from cppTango. So if one has a better description, you are welcome here
  3. I added some tests. However, I am not sure if I use it in proper way. It will be cool, if cppTango has somewhere description and examples. It is not trivial to realize, how to use it just looking to the code
Edited by Yury Matveev

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