Prevent extend_enum if pollutant name already exists

[stroutm]

This happens if OutputMetadata is instantiated more than once.

If symbolic_name is already present, an error will be thrown.

[stroutm]

Opened PR

[bemcdonnell]

Hey @stroutm! Great to see you in the code. And thanks for sharing your fix! Can you show a little but of the code you were using that threw the error? I looked at the PR for a minute and it's making me think of something else related to this.

[stroutm]

I am trying to read from the SWMM out file more than once, in which output_metadata.OutputMetadata(p_handle) is called each time. Doing so results in the following traceback: