Clock and trigger controls

[guruofquality]

Referencing this issue, SoapyBladeRF should support the clock source API and trigger source API so it can be used to sync multiple bladerfs -- and be used in SoapyMultiSDR.

• Use the setClockSource() family of calls to wrap the reference clock master/slave api
• Use the setTimeSource() family of calls to wrap the trigger signal config API
• Using the new SOAPY_SDR_TRIGGER flag to cause the trigger when working with the streaming api. This usage is documented with the trigger signal api as an example.
• Use the setHardwareTime(time, "TRIGGER") API to initiate the trigger, automatically sync'ing the time offsets. This will work a lot like the existing BLADERF_GPIO_TIMESTAMP, but for multiple boards. Internally, the boards must do a receive to get the latest timestamp and adjust the offset. This process will have to re-do itself automatically upon sample rates changes (also like the current BLADERF_GPIO_TIMESTAMP method).