Permit replacement of pin_factory without closing old factory. However,
continue closing devices associated with extant pin factory at script
termination.
Doh! SPI tests fail when the spidev package is installed (which it
normally isn't in my virtualenv) because hardware SPI always takes
precedence and the mock SPI stuff only replaces software SPI.
Sorry! Dave's messing around with the pin implementations again.
Hopefully the last time. The pin_factory is now really a factory object
which can be asked to produce individual pins or pin-based interfaces
like SPI (which can be supported properly via pigpio).
Added SPI tests, simplified the shared SPI software bus implementation,
and fixed several protocol errors in our MCP3xxx classes (the x2 and x1
protocols were wrong)
