v0.4.0

SoapySDR v0.4.0

Diff since v0.3.4

Merged pull requests:

• Add error checking to all SoapySDRDevice_ calls (#75) (@staticfloat)
• Add group activate/deactivate (#76) (@staticfloat)
• WIP: add high level interface for clock sources (#79) (@sjkelly)
• Error Handling on Read/Write (#80) (@sjkelly)
• support enumerating Register, UART, and GPIO interfaces (#81) (@sjkelly)
• Highlevel Register interface improvements (#82) (@sjkelly)
• A few minor cleanups (#83) (@sjkelly)
• Misc improvements (#84) (@sjkelly)
• Add Support for Channel Settings with High Level interface (#85) (@sjkelly)