backwards compatibility for UVPSpec.r_params

[nkern]

The lazy covariance PR added the UVPSpec.r_params attribute, which is generally a dictionary encoded in a JSON string, but we didn't add an exception for reading UVPSpec objects from disk that don't have r_params attribute in their file structure (i.e. files older than the PR itself). @steven-murray alerted us that this was causing hera_stats tests to fail, which does store a static uvp for its testing suite.

This wasn't caught in our normal review process because we don't store static UVPSpec files in our testing directory. #219 addresses this issue specifically with r_params being backwards compatible, but also adds a single, simple static UVPSpec object to the data directory for testing backwards compatibility in the future.