Ensure Ei values are stored updated and used correctly

[Nick Draper]

Both ConvertUnits and GetEI should be checked.

GetEI should always store the value in the run object.

ConvertUnits should also store it if it was explicitly added as a parameter (overwriting if already present). ConvertUnits should use the stored value if it was not explicitly provided.

[Michael Whitty]

(In [6474]) re #1684 - ConvertUnits will now take the Ei value from the run object if none is provided, will store the input if a value stated by the user, or will throw an exception (std::invalid_argument) if it can't find a value for it at all.

Same changes made to the event workspace convertViaTOF - however would suggest someone tidy up as there's a lot of code copy/paste between the two at the moment.

[Michael Whitty]

(In [6479]) re #1684 - hopefully fix tests

[Martyn Gigg]

(In [6500]) Minor update to ConvertUnits and GetEi to ensure that if Ei already exists, then it is overwritten. Re #1684

[Michael Reuter]

The code does work. I've created a ticket about code consolidation in #1833.

[Stuart Campbell]

This ticket has been transferred to github issue ​2531