fix(hRIXS): drop non-existing mnemonics and add attributes on dataset

This MR accounts for supplied mnemonics that are non-existing for the MaranaX.centroid_from_run method.

I also sneakily added some attributes that describes the resulting data.

@teichman @kuikenb

doc/changelog.rst

@@ -19,6 +19,7 @@ unreleased
     - adds MaranaX mnemonics :mr:`285`
     - adds Chem diagnostics and Gotthard II mnemonics :mr:`292`
     - make hRIXS centroid threshold configurable :mr:`298`
+    - drop MaranaX non-existing mnemonics and add attributes on dataset :mr:`300`
 
 - **New Features**