I chose the version from the main repository for two
reasons:
- Explicitly specifying si5324_ref_input every time would
not work for the different Kasli/… hardware versions.
- Having `crystal_ref` as a setting in the configuration
is misleading if it does not actually activate the crystal
for use as a reference (but rather does
`route_crystal_to_ckin2`).
Related m-labs/artiq commits:
- 740543d4e284245248e3ff838c46505938dcae7a
- 3c7a394eff553ab75a7ea78bdd17830366504dc6
5c054cc901