SP3 clock normalisation & Helmert transformation

[EugeneDu-GA]

Enable clock normalisation and Helmert transformation with sp3_difference

[ronaldmaj]

This looks good to me. I've done a check for other functions that call gn_io.clk.sv_clocks but I don't see anything else.
There is already a unit test for compare_clk() so that's covered, but I cannot say anything about sp3_difference() or clk_difference(). If it's working as expected for your purposes then I think that's fine.
I would look at putting through unit tests for these two functions (doesn't have to be in this PR)

[ronaldmaj]

As I said before, I think this is in a good enough state to go in, but I would wait on @seballgeyer to give his final approval before merging in

The one thing I would like to see is unit tests on functions that were touched here, but perhaps that could be done in a more wide-ranging PR that creates unit tests for a whole range of functions