#280 S/N estimator set and configure

[Willem van Straten]

The current interface to set the algorithm used to compute the S/N is inflexible and not configurable. It currently uses the Pulsar::Option template with an embedded SNRatioInterpreter object that parses the text; however, it would be better to use the TextInterface::factory template function, which also allows the S/N estimator to be configured via its TextInterface.