This commit responds to the code review comments:

- Extract constant strings which are used multiple times into const string variables
- Throw immediately if multi period.
- Remove unclear call to getProperty("OutputWorkspace) in LoadMuonNexusV2
- Remove unnecessary includes and in doing so create forward declarations for TableWorkspace and Workspace2D
- Use std::move for string and workspace copying.
- Remove unnecessary pointer casting, which will be dealt with by the compiler.