Add interrupt support & examples to MCP23017

[ladyada]

this is for a gpio expander bonnet, can now manipulate the IRQ control registers. also set default to be IRQ pins are open drain.

[siddacious]

Code looks good but I don't have the hardware to test so I'll leave it to you to merge or for someone else to test if need be.

As a general comment about this lib I am kinda 🤷‍♂️ about exposing lightly wrapped registers; on one hand it's great for those who know what they're doing, but maybe not so useful for fresh circuit pythonistas? Then I suppose that ship has sailed.

[ladyada]

normally i wouldnt...i guess i could try to abstract em but it'd get really 'wordy' - this was to test a new hardware.

[tannewt]

+1 to renaming the properties to fully explain what they are. The comments are really handy so by renaming we'll remove any hint of data sheet-ism.

[ladyada]

but, they'd still need to be assigned and read as if they were the registers... zat ok?

[tannewt]

Sorry it took me a while to get back.

Ya, I think it's fine that they are bitmasks. The documentation makes that clear. The fact that they map to a register under the hood isn't important though.

[ladyada]

added/updated!
take a look

[ladyada]

@tannewt ok im gonna merge this - next time i sweep thru this library ill update them to registers, i think perhaps this was written pre-register