Better documentation

[jschaeff]

I updated the openAPI specifications:

• review of the descriptions
• change of the server / URL
• adding a _nodes method to get list of nodes (needs to be implemented)
• set datacenter parameter as an array, which enum values are among the /_nodes result (need to be finished)

@vpet98 could you:

• look at the best way to generate the documentation of the webservice directly ? Like for exemple with https://pypi.org/project/openapi-ui-bundles/
• finish the work on the API description by adding default values, specifying when multiple values are possible (look at the openapi spec)
• implement _nodes method
• rename health to _health
• enable '*' wildcard on Net/Sta/Loc/Cha selection parametes
• datacenters : set '*' as default value or put in description that empty means all datacenters.

[vpet98]

The way it is implemented, leaving for example station field empty means all stations, like in fdsn webservice. Shall I change this so that leaving station empty means a default station e.g. CIEL? The '*' wildcards are enabled.

Also, shall I add a format parameter at /dataselect/stat method as you added at specs?