Improve check valve validation and setting

[angusmcb]

Provide a summary of the proposed changes, describe tests and documentation, and review the acknowledgement below.

Summary

Include issues that are resolved by this pull request.
Improved method of setting check_valve on pipes. Validation and conversion from string values 'True', 'False', '1' and '0' occurs within element rather than with add_pipe. This means when check_valve is set directly validation will occur.

Contributres to #422. This is also a step towards allowing use of geopandas >= 1.0. This is because shapefile booleans seem to be returned as strings 'True' and 'False' rather than strings '1' and '0' so current normalisation with check_valve = bool(int(check_valve)) fails. This resolves the following error when trying to use geopandas >= 1.0

>       check_valve = bool(int(check_valve))
E       ValueError: invalid literal for int() with base 10: 'False'

May also fix #475

Tests and documentation

Does not change the API functionality, covered by existing tests.

Acknowledgement

By contributing to this software project, I acknowledge that I have reviewed the software quality assurance guidelines and that my contributions are submitted under the Revised BSD License.

[angusmcb]

I believe the test fails are not related to the changes made

[angusmcb]

Closing in favour of #488 which contains these changes but is more complete allowing the latest geopandas to be used.

[coveralls]

coverage: 81.805% (+0.02%) from 81.788%
when pulling 0584c2e on angusmcb:improve-check_valve-validation
into 88ca1d9 on USEPA:main.