permit loading a .sumocfg

[namdre]

• new option --sumocfg FILE
• load all files referenced in the configuration (with the appropriate handlers)
• allow manipulating all options (probably by parsing a configuration template in order to build a configuration wizard - see add wizards for python tools #4138)
• write all these options when using menu option to "Save SUMO-config"

[palvarezlopez]

For edit sumocfg options we can use #2435

[namdre]

I think we shouldn't mix a property dialog with an options editor.

• The network properties are just a collection of computed properties (i.e. total edge length)
• The sumo configuration should be edited in the same manner as it now happens with the netconvert options (F10)

[palvarezlopez]

Use options handlers (template)