replaced measurements with features for bulk upload
fixed track decoding
fixed NPE with missing Content-Type header
added unit for measurement values (closes #27)
fixed group creation validation
Fixed unintended recursion (closes #45)
Merge branch 'dev' into activities

* dev:
  Fixed unintended recursion (closes #45)
  fixed group creation validation
  added unit for measurement values (closes #27)
  fixed NPE with missing Content-Type header
  fixed track decoding
  replaced measurements with features for bulk upload

Conflicts:
	core/src/main/java/io/car/server/core/Service.java
	rest/src/main/java/io/car/server/rest/encoding/MeasurementEncoder.java