add example: reading a GPIO input with the ULP coprocessor

[wnienhaus]

This PR adds a simple example for reading a GPIO pin with the ULP coprocessor.

When running this, the ULP will read the current GPIO value (0 for off, 1 for on) into a memory location, and a loop at the end of the example will continuously print the value in that memory location. This allows seeing the value update as one toggles the input to the GPIO pin between HIGH to LOW (e.g. by connecting and disconnecting a wire from VCC (3V3) to GPIO4).

[ThomasWaldmann]

did you try the code?

[wnienhaus]

did you try the code?

Yes. I did. Tried it on a Lolin32 clone, with GPIO4 either connected to VCC or open, and the output of the python while loop printing 1 while connected to VCC and 0 otherwise. I have a more elaborate version of that code running "in production" in a rain-gauge I built (using this board), but this example simplifies it down to the bare essentials for reading the GPIO.

I have also asked @kjm1102 to try this example, as we together worked through various details to get it all to work on his ESP32 at all (long discussion in issue #70). The example is the result of that discussion, with the hope of saving the next person the time to figure it out, or having to read through #70 :).

I'll add a comment to this PR, once @kjm1102 has tested this example on his ESP32.

[wnienhaus]

As per @kjm1102 (mentioned in issue #70):

The #73 demo program works great, displays 0x0 (gpio4 not connected to anything) or 0x1(gpio4 bridged to 3v3).

So, I guess we can merge.

[wnienhaus]

Thank you!