Encoder: initialize only the given channel

[dplanella]

Currently, upon instantiating the RotaryEncoder class, a channel index can be given to initialize and use a particular eQEP channel.

On initializing the class however, all possibe channels are configured as qep, regardless of whether they are available or not. That is, the channel index is ignored and all channels are configured instead of just the given one.

This poses a few issues:

• All QEP-enabled pins are inadvertently configured as qep, overriding their global setting, or their configuration in other parts of the code
• Even if a pin is not exposed, the RotaryEncoder class will try to initialize it, resulting in an error
• If both the eQEP2 and eQEP2b channels are available, they are also both configured in sequence. Given that they are mutually exclusive, if eQEP2 is configured first and eQEP2b is configured afterwards, eQEP2 will be inadvertently disabled.

This change addresses these issues by configuring only the given channel upon initialization of the class. Public module constants have also been added for easy reference to each channel.

A quick example of using the code with the new changes:

import Adafruit_BBIO.Encoder as Encoder

'''
Each channel can be accessed and initialized using its corresponding
channel name constants:

    Encoder.eQEP0
    Encoder.eQEP1
    Encoder.eQEP2
    Encoder.eQEP2b
'''

# Instantiate the class to access channel eQEP2, and only initialize
# that channel
myEncoder = Encoder.RotaryEncoder(Encoder.eQEP2)

[pdp7]

Thanks @dplanella