ModelData Utilities

[darrylmelander]

Summary/Motivation:

These utilities are useful for Prescient when working with ModelData objects, and may be useful to others as well.

Changes proposed in this PR:

Added two utilities that help you work with ModelData objects:

• egret.model_library.transmission.tx_utils.element_types() - Returns an iterable that lists all valid types you can pass to ModelData.elements('type-name')
• egret.parsers.rts_gmlc.parser.ParsedCache.get_timeseries_locations(ModelData) - Returns an iterable that enumerates all the places in the passed-in ModelData object that holds a time series, as specified in the RTS-GMLC data that was parsed and cached in the ParsedCache object.

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[bknueven]

Looks good, besides correcting a mistake I made ...