Allow user code to set W element of RGBW pixels via pack() function (issue #25)

[PaintYourDragon]

This is NOT True and Robust RGBW Handling™, but does allow easy setting of the W element of RGBW NeoPixels without having to unpack() a previously-pack()ed result. The pack() function accepts an optional W value, in which case it returns a 4-tuple that can be used with the NeoPixel() setter. This is esoteric but the need for it is discussed in issue #25, due to the way the CircuitPython NeoPixel setter works (tuples are preferred to packed ints). This provides a relatively benign quick compromise without having to implement a whole CRGBW class.

There’s some notes in the library, but here’s an example of it in use (from the fancyled_neopixel_rotate_simpletest.py example, adapted for RGBW)…this shows how if the RGB color gets gamma-corrected, the W value must be separately gamma-corrected before merging it in via pack(). Produces an RGB swirl with a white ramp overlaid.

import board
import neopixel
import adafruit_fancyled.adafruit_fancyled as fancy

num_leds = 30

# Declare a 6-element RGB rainbow palette
palette = [
    fancy.CRGB(1.0, 0.0, 0.0),  # Red
    fancy.CRGB(0.5, 0.5, 0.0),  # Yellow
    fancy.CRGB(0.0, 1.0, 0.0),  # Green
    fancy.CRGB(0.0, 0.5, 0.5),  # Cyan
    fancy.CRGB(0.0, 0.0, 1.0),  # Blue
    fancy.CRGB(0.5, 0.0, 0.5)]  # Magenta

# Declare a NeoPixel object on pin D6 with num_leds pixels, no auto-write.
# Set brightness to max because we'll be using FancyLED's brightness control.
pixels = neopixel.NeoPixel(board.GP2, num_leds, brightness=1.0, auto_write=False, pixel_order=neopixel.GRBW)

offset = 0  # Positional offset into color palette to get it to 'spin'

while True:
    for i in range(num_leds):
        # Load each pixel's color from the palette using an offset, run it
        # through the gamma function, pack RGB value and assign to pixel.
        color = fancy.palette_lookup(palette, offset + i / num_leds)
        color = fancy.gamma_adjust(color)
        # Insert a gamma-corrected W value at the end before assignment
        pixels[i] = color.pack(fancy.gamma_adjust(i / (num_leds - 1)))
    pixels.show()

    offset += 0.02  # Bigger number = faster spin
    print(offset)

Also took care of some minor refactoring that was mentioned as a TODO in there.