Require classification metadata in YAML configs

[aromanielloNTIA]

Addresses #41 by requiring action configs to specify the classification status of the data the action will produce. This value will be written to the SigMF ntia-core Measurement Object in the metadata produced by the action.

Existing action configs will need to be updated to avoid errors, since time domain IQ actions and the M4S FFT action now both require a value for classification in their configs. Y-Factor action configs do not require a classification value, since they do not save SigMF metadata.

Previously, results from these actions had no classification recorded in their metadata results, despite this technically being a required key according to the ntia-core specification. Since #32, classification has been hard-coded as UNCLASSIFIED.