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)

[Robadob]

iirc this is because macro properties have to be set during runtime (e.g. in init function or agent function), they never exist on the host. (Somewhat arbitrary development decision, to avoid people doing bad perf stuff)