Model summary property and RPT warning function (#154)

[everettsp]

• Created summarize_model function and _summary model property. The summarize_model() function can very easily be expanded to include more fields. This could maybe be changed to a function in the future to add the option of a more detailed summary
• Created rpt_warnings function to read warnings from the head of the RPT file. The function requires a valid RPT file, but the run itself doesn't need to be successful. For warnings beyond those included in the RPT file, looks like drop_invalid_model_elements() currently provides some geometry warnings.

[aerispaha]

@everettsp thanks for this contribution! I'd like to add some unit tests and a bit of documentation for this, but we can capture that in a separate issue.