Provide location-based query for repeater

[smangels]

• returns a list of repeater matching max distance from a given geographical position
• add function test case for this query
• reduce technical debt in function test
• MongoDB, create "2dsphere" index in "loc"
• Harvester, ensure that all entries are scanned and a new "loc" object is created when geographical position is valid
• make use of Falkenberg position and distance of 30km as default value
• closes #71