Oh, I thought udunbits 2 already did that for us but it doesn't.
```
>>> import ctypes
>>> from iris.unit import Unit
>>> _lib_ud = ctypes.CDLL("/usr/local/sci/lib/libudunits2.so", use_errno=True)
>>> greg = Unit("days since 2001", calendar="gregorian")
>>> stan = Unit("days since 2001", calendar="standard")
>>> julian = Unit("days since 2001", calendar="julian")
>>> _lib_ud.ut_are_convertible(greg.ut_unit, stan.ut_unit)
1
>>> _lib_ud.ut_are_convertible(greg.ut_unit, julian.ut_unit)
1
```
It seems to think it's ok to convert from greg to julian...
I'll close this, pending re-work.