Add expectedExptime=SS.SS argument to the `hx ramp` command

We do not yet have fleshed out mechanisms either for calling the hxActor just before the end of the exposure or for back-patching the PHDU.

Accept an `expectedExptime=SS.SS` argument to the `ramp` command, and use that value to set the EXPTIME card for 'arc', 'flat', and 'object' .