Add correct support for grouping of data on graphs on ndoewatcher

[SloMusti]

Nodewatcher supports grouping measurements on a single graph with the setting group, putting all measurements on a single graph that have a matching value set.

Should there be a group field in EasyEsp, that would be straightforward, however that would require the whole system to be changes and likely we can reuse some other field.

Every device is defined with Name and IDX / Var and name should be different for all sensors. Note that display or other things may assume each sensor has a different task 'name' and thus we can not simply use name as a group and idx as sequence number.

Lets consider a few EspEasy specific examples:

1. sensor with one value, simple, define each device separately, give them common group naming to group
2. sensor with multiple values, we want all the values to be in the same group or graph
3. sensor with multiple values, we want to have values on separate graphs but we may want to group a number of the same sensors together

We have a number of options:

1. Choose the device name to be unique, auto-number all sensors internally and use IDX value as group selector. Note IDX value must be non-0 and can not be text. Breaks the use-case 3, because IDX is per device not per value.
2. Construct the device name to represent the group and then use idx to be the unique identifier. This breaks the way display is implemented as well as use-case 3.
3. Define group per value directly with a delimiter, for example <readable name>,<unit>,<group> and if group undefined, do not define it. Does not break any use-cases, makes usage with displays a bit less practical, but still feasible.