Make diagrams configurable per context (per period) / Refactor diagrams configuration

[Daveiano]

Currently, the skin only allows to define the diagrams once, this definition is then used on all pages (index, week, month, year, statistics). So it is not possible to show a specific diagram e.g. the outTemp min/max/avg diagram only on the statistics page. Every diagram is shown everywhere.

This should be configurable like in the default seasons skin [ImageGenerator] section:

[[day_images]]
        x_label_format = %H:%M
        bottom_label_format = %x %X
        time_length = 97200 # 27 hours
        
        [[[daybarometer]]]
            [[[[barometer]]]]
        
        [[[daytempdew]]]
            [[[[outTemp]]]]
            [[[[dewpoint]]]]
...
[[week_images]]
        x_label_format = %d
        bottom_label_format = %x %X
        time_length = 604800 # 7 days
        aggregate_type = avg
        aggregate_interval = hour
        
        [[[weekbarometer]]]
            [[[[barometer]]]]
        
        [[[weektempdew]]]
            [[[[outTemp]]]]
            [[[[dewpoint]]]]
...

Things to consider
- Refactor getDiagramProps methods to use weeutil.config.accumulateLeaves and search_up
- Expose enableArea and areaOpacity in skin.conf for diagrams

Related issues:
#25
#71
#75