Allow arbitrary clock referencing strategies

[glopesdev]

To allow arbitrary referencing of the Harp data clock may require custom implementations to traverse the diverse object types which may represent a stream data attribute.

To allow for greater flexibility of implementations, e.g. in the case of EEG where we don't even control the reader class itself, this PR refactors the reference_harp_to_ubx_time method to use polymorphism, by introducing a new rereference_clock_origin in the base Stream class. This allows implementations to specify how exactly to apply a new arbitrary clock offset to the stream data.