Provide [enumeration name]AsString for all enumerations

[hsorby]

We have some [enumeration name]AsString static methods but not all.

I have found these enumerations to have a conversion function:

• CellmlElementType —> cellmlElementTypeAsString
• AnalyserEquation::Type —> typeAsString
• AnalyserEquationAst::Type —> typeAsString
• AnalyserModel::Type —> typeAsString
• AnalyserVariable::Type —> typeAsString
• GeneratorProfile::Profile —> profileAsString

I have not found a public static conversion function for these enumerations:

• Issue::Level
• Issue::ReferenceRule
• Units::Prefix
• Units::StandardUnit
• Variable::InterfaceType

I think this is all of the enumerations available throught the public API.

This issue is to provide the static conversion functions for all the enumerations that cannot currently be converted to string.

This would be good for completeness and consistency more than anything else.