v0.5.3-beta

• Added compatibility with OpenMDAO 3.4, which is now the minimum required
  version of OpenMDAO. (#231)
• Simplified call to VariableViewer. (#221)
• Bug fix: model for compressibility drag now takes into account sweep angle
  and thickness ratio. (#237)
• Bug fix: at installation, minimum version of Scipy is forced to 1.2. (#219)
• Bug fix: SpeedChangeSegment class now accepts Mach number as possible target. (#234)
• Bug fix: variable "data:weight:aircraft_empty:mass has now "kg" as unit. (#236)