Merge pull request #491 in VECTO/vecto-sim from master to develop

* commit '0e934967':
  use FLD values if declared speed/power are 0
  formatting xml output
  update resource file
  computing average rolling resistance
  correct testcase
  add attributes to air density property: required
  adapt testcases related to airdrag: pass along air density; set air density in model data in testcases
  add air density (and gravity acceleration) as constant to declaration data. add air density to vehicle data, use air density from model data for air drag computation declaration data adapter: use declaration data; engineering data adapter: use physics constant to fill model data GUI: set air density (physical constant) in declaration mode
  adapt output schema to include AverageRRC
  move strings to resource, add averageRRC to customer report
  culture-independent decimal separator for writing SI values to XML format