[#508] Compute lat/lon of clusters as an average over all points
[#509] use clusterId as keyId to uniquely identify clusters.

* use new detailsId field to convey the surveyedLocaleId to the client.
[#510] Make placemark keyId unique across clusters and locales
[#508] Use primitives for constants
[#363] Move adapt cluster data to task
[#363] Remove unused code
Merge pull request #511 from akvo/hotfix/1.6.14

Hotfix/1.6.14
[#512] Release notes 1.6.14.1
Merge branch 'release/1.6.14'
[#363] Change levels at which different clusters are shown
Merge pull request #513 from akvo/1.6.14/hotfix

[#363] Change levels at which different clusters are shown
[#514] Release notes 1.6.14.2
Merge pull request #515 from akvo/1.6.14/hotfix

[#514] Release notes 1.6.14.2
Merge branch 'release/1.6.14'
[#516] Check for lastSurveyalInstanceId not null

* in old data, this field can be null. In that case
the data will be considered private and included
in the private clusters.
[#518] Release notes 1.6.14.3
Merge pull request #517 from akvo/1.6.14/hotfix

[#516] Check for lastSurveyalInstanceId not null
Merge branch 'release/1.6.14'
Merge branch 'master' into develop