Remove Base Encoder.formatBits() function

[rhyolight]

This function is defined on the base encoder class, so is available for all encoder subclasses, but is never called within our codebase. It seems to be entirely for displaying SDRs in a console log.

https://github.com/numenta/nupic/blob/8d97b2b1e8be99fbb6d753ddca1e168e4c416028/src/nupic/encoders/base.py#L770-L791

I suggest we remove it entirely, for several reasons:

• it is not used anywhere
• there are no examples of usage
• documentation is incomplete
• it doesn't work

I tried to use it, and it did not work as I expected when I added more than one sub-field in the encoder description. This script yielded an error:

[('test formatBits 1', 0), ('test formatBits 2', 3), ('end', 10)]
Copying: [1, 0, 1]
Traceback (most recent call last):
  File "format_bits_test.py", line 30, in <module>
    formatBits(width, desc, input, output, scale=1)
  File "format_bits_test.py", line 23, in formatBits
    outarray[end+i+leftpad] = blank
IndexError: list assignment index out of range

(I printed out the description before it was looped.)