Add support for calibration uncertainty.

Add support for injections which contain a calibration uncertainty. This would involve adding a number drawn from a Gaussian distribution to various parameters of the injections. For this to work well it should be developed after or in tandem to a fix to #23, since being able to specify a calibration uncertainty in the phase might be important.

@james-clark Suggested comparing the approach x-pipeline has taken to this problem in the past; their injection code is here

• Decide on correct approach to this in the API
• Write unit tests for new behaviour
• Implement the addition of uncertainties