While working on issue #1244, I can see that some of the API for external variables could be improved:

• bool addExternalVariable(const AnalyserExternalVariablePtr &externalVariable)
  An AnalyserExternalVariablePtr is currently needed, which is needed when an external variable has some dependencies, but not if it doesn't have any. In the latter case, we should be able to add an external variable using just a VariablePtr.
  • Add bool addExternalVariable(const VariablePtr &variable)
• bool removeExternalVariable(const ModelPtr &model, const std::string &componentName, const std::string &variableName)
  Rather than passing a ModelPtr, a component name, and a variable name, we could simply pass a VariablePtr.
  • Replace with bool removeExternalVariable(const VariablePtr &variable)
• bool containsExternalVariable(const ModelPtr &model, const std::string &componentName, const std::string &variableName)
  Same as above.
  • Replace with bool containsExternalVariable(const VariablePtr &variable)
• AnalyserExternalVariablePtr externalVariable(const ModelPtr &model, const std::string &componentName, const std::string &variableName)
  Similar to above.
  • Replace with AnalyserExternalVariablePtr externalVariable(const VariablePtr &variable)