Added GPS information and 

Main Changes 
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- introduced GPS information
- introduced some of the missing connections between the preferences and the actual code
- modified the way data is uploaded to the server:
     1. not every reading has long/lat and time. This is a waste of data upload. Data is compacted into a common structure and then it is uploaded in one go when completed
     2. taken care of cases where the server is not responding: created a local database where data is temporarily stored. Data will be sent next time the app is loaded
- the interface does not have a table with all data but a textview with only the current data: this is much easier to read
- the font color has been changed to greyish to white (I could not read it)
- the names of the views in the interface have now a positional name (row1_column2). This is because in the future it would be great to have the allocation of data flexible (so that you can change what is displayed where
- other minor changes

Issues
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The data displayed is wrong. This is not an issue with this branch. It is something that happens also in the original repository.