Missing `MacroProperty` methods in `EnvironmentDescription`

[ptheywood]

The EnvironmentDescription class only contains the ability to add new MacroProperty values, via newMacroProperty.

There are no methods to get an existing MacroProperty (i.e. no getProperty equivalent.

A method for checking if a macro env property has been set would also be good, to standardise the description interfraces with AgentDesction a little (aslo see #1218)