Decodes an EmbracePlus .avro file into a
PhysioCore::MultiRatePhysioExperiment. Each signal in the record's
rawData (EDA, BVP, temperature, accelerometer) becomes a stream at its
samplingFrequency, aligned by its timestampStart (microseconds). Requires
an Avro backend (the Python fastavro module via reticulate; see
hasAvroBackend()).