Also added updates to user documentation, and unit test for histograma and event workspaces.

The main reason for this change is that EventList does not contain Y
data, so we cannot give write access to Histogram via ISpectrum (except
for the X data).

re #15450

re #15289

The initial value had the wrong type so each additional was converting
the double to an int leading to the incorrect result.
Refs #14129

Refs #13262