exclude battery charging times from guesstimated battery lifetime

[individual-it]

in case the battery is recharged the prediction values become useless. e.g. time now-14days the battery was 60%, during the next 7days it went down to 10%, got recharged to 100% and is now standing at 59% for the time-frame of 14days the current algorithm would only check the oldest and newest data point, seeing 1% of discharge over 14days and so calculating an useless high "time left" value. Alternative approach would be reading every value backwards from the latest one and adding up all the discharge values, ignoring times when the battery was charged.

Advantages:

• better prediction

Disadvantages:

• longer time to calculate the result (reading out of the storage takes time - see the time it takes to create the graph)
• small charges by solar would not be taken into consideration for the prediction

[individual-it]

this could be triggered by the GPS button when in that view or by a setting

[individual-it]

as the screens are switched around now, see #18 using the GPS button would not be good easy solution: show this calculation on the graph screen, there the data anyway is already read from permanent storage, so that is a big time advantege