add energy offset to baseline

[ambarb]

it is already there for tardis, and tardis actually has corrected energy on top of that. this could be messy because it depends on enforcement. if this is chosen, we need to make a function for people to use when the change the tardis.calc energy. @cmazzoli probably worth a discussion

@johnsinsheimer does the new IOC that you are working on for IOS flyscanning have a PV to be used as a constant offset for exact energy calbration (so the readback + offset). Typically, changing the gratings can create 1 -3 eV shift that does not change the resolution.