Fix reader's association with file extensions in the GUI dialog.

Readers that were available through the GUI file open dialog
were specified based on the client side build, including plugins.
So even though a reader on the server may be available it was
the client that was determining which readers it thought should
be used for opening a file on the server. Thus, if any reader
depended directly on MPI it wouldn't be available on the client
if the client was built in serial. This change fixes that so that
the assocation of file extensions to readers was based on the
server's servermanager XML.

Change-Id: Ie7cf4fd3caaa3bd2dfcf17a33baa5b9c69072af5
Changing reader plugins to incorporate being only needed on the server.

In the previous commitI made it possible for reader plugins to
work properly when it is only defined on the server. Now I'm
changing the plugins (besides the nektar reader plugin) to
take advantage of that.

Change-Id: Ibf7836fbc54e9cd1fed192203f48a162ccd98a14
Fixes to vtkSMReaderFactory.

Added the ability to add and remove groups to search for readers
in. Also fixed a Python bug for a class that got removed.

Change-Id: Ifbfca572e5a473363e6da3a7a302424dc2246fc8
Changing the writers to be available on the client based on server XML.

Same changes that I just did for readers to make them only defined
on the server and then pass that info to the client. Also removed
the GMVReader plugin's GUI XML that isn't needed. Finally, added
in warnings for both reader and writer plugins that still define
the GUI XML stuff instead of putting the hints in the server
side XML Hints section.

Change-Id: Idbf592f0cebcb69a214e1677743cf64318110380
Merge topic 'serverside_only_reader' into next

d23fc9e Changing the writers to be available on the client based on server XML.
7a5de17 Fixes to vtkSMReaderFactory.
5627d44 Changing reader plugins to incorporate being only needed on the server.
e675269 Fix reader's association with file extensions in the GUI dialog.