Refs #14838

Hopefully, it's a clearer and more concise version of what Anders requested.

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

 1. Change the filename reported for nexus files if xml contained within
 1. Stop saving the filename to nexus processed files
 1. removed some unused includes

Still to do:
 1. cache SHA1 checksum if needed
 1. Manually test
 1. update documentation
 1. investigate this error:

ctest -C Debug -J 4 -R LoadEventNexusTest -V

 548: LoadParameterFile-[Error] Bad path syntax: C:\Mantid\Code\Mantid\instrument
\C:\Users\rrc79113\AppData\Roaming\mantidproject\mantid\instrument\HYSPECA_Param
eters.xml. Unable to parse File: in C:/Mantid/Code/Mantid/instrument//C:\Users\r
rc79113\AppData\Roaming\mantidproject\mantid\instrument\HYSPECA_Parameters.xml
548: LoadIDFFromNexus-[Notice] Instrument parameter file: C:\Users\rrc79113\AppD
ata\Roaming\mantidproject\mantid\instrument\HYSPECA_Parameters.xml not found or
un-parsable. Instrument parameter file: C:/Mantid/Code/Mantid/instrument/HYSPECA
_Parameters.xml has been loaded

re #11818

`openLocal` performs far better than open, and we're able to use it when
loading multidimensional data sets in LoadNexusesProcessed, so let's.

It is still backwards compatible with the old log-entry format.
Refs #11359