Particular attention was paid to overwriting or not of existing logs.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>

Try to handle case where there is no proton charge in a particular entry.
Made the error message more user friendly.
In LoadEventNexus I changed the order of parameters, so that NXentryName is closer to the top

re #15289

in all cases call getProtonCharge to get the proton charge
re #14323

We only need to load the logs for the measurement information.

Create synthetic nperiods log entry. The nperiods entry is present in the file, but not in the logs section. Having this will make the logs given to LoadEventNexus contain the same nperiods entry we currently have on the histogram workspaces.

The converted text is stored in separate .rst files in the docs directory.
Refs #9562

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>

LoadNexus, LoadNexusLogs, LoadNexusMonitors, LoadNexusProcessed,
LoadPDFgetNFile, LoadParameterFile, LoadPreNexusMonitors,
LoadRKH, LoadRawBin0, LoadRawSpectrum0, LoadRawHelper

re #6485