Replace ToolMode with `toolProperty`

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Instead of using the ToolMode enumeration, consider having toolProperty, whose values would be the 3 tool model elements:

• model.tools.conductivityTester
• model.tools.pHMeter
• model.tools.pHPaper

There would need to be a Tool base class, with ToolIO reference type serialization.

This might be more work than it's worth, but I'll ruminate.

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I decided not to do this. It's too much work for almost no benefit. Closing.