Updates for dual channel support

[caternuson]

This is for #20.

A couple of significant changes

• Removed enabling of external PGA filters in ::begin() since this is only applicable to single channel configuration (i.e. the original version of PID 4538). This has been moved to new function setPGACap() which can be called from user code, as shown in new nau7802_single_channel example.
• The two new examples do not call the calibration functions. The behavior of these functions is confusing in the two channel scenario.

Tested with a rev C two channel version of PID 4538 on a Qt PY M0 and two 10kg strain gauges.

Example output from nau7802_dual_channel example:

[caternuson]

@ladyada for review