Some of the LET files have all zeros for the period number. We fix these
to be 1.

Fix warnings. Refactor DecoratorWorkspace into it's own physical unit. LoadEventNexus is already too big! A few tests added to that type in the process and forward declares used.

Rough tests of output.

We do not allow loading of monitors in event mode for multiperiod event data.

While maintaining backwards compatibility, ensure that any action done on our Decorated workspace is now done on the complete set of workspaces we have.

Decorator now holds vector workspaces corresponding to periods.
Decorator initializes vector of workspaces once n-periods has been determined.

We want to know where we are explicitly accessing the held single member workspace, as this is going to change and will otherwise be the source of bugs.

Before I convert this to a vector of workspaces, I notice that the same variable name WS is used both for local variables, and member variables. This is utterly stupid and bound to cause problems in the future, so i'm fixing the naming here first.

re #11818

It wasn't adding spectra to the workspace.