Update MSSChemDataAccess from example configuration in documentation

[Andreas Hilboll]

The docs/deployment.rst file contains an example mss_wms_settings.py file. This still gives an example on how to use the MSSChemDataAccess class, which was removed in 1.6 (?).

The documentation should be adapted accordingly, to show usage examples using the new DefaultDataAccess class. This should be done for 1.6.x.

[Reimar Bauer]

good catch :) pull request welcome :)

[Andreas Hilboll]

see pull request #492

[Reimar Bauer]

DOC: updated documentation for MSSChem classes in WMS settings

fixes issue #291

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