allow math avg to work with durations (#11598)

# Description

I ran into a problem where one of our benchmark tests in nu_scripts
wouldn't work because math avg wouldn't work with durations, so I made
these changes to support it. I'm confident that there are other math
commands that probably need this "fix".

Side note - we should really fix our inout_output_type system.

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crates/nu-command/src/math/avg.rs

@@ -18,9 +18,12 @@ impl Command for SubCommand {
     fn signature(&self) -> Signature {
         Signature::build("math avg")
             .input_output_types(vec![
-                (Type::List(Box::new(Type::Number)), Type::Number),
                 (Type::List(Box::new(Type::Duration)), Type::Duration),
+                (Type::Duration, Type::Duration),
                 (Type::List(Box::new(Type::Filesize)), Type::Filesize),
+                (Type::Filesize, Type::Filesize),
+                (Type::List(Box::new(Type::Number)), Type::Number),
+                (Type::Number, Type::Number),
                 (Type::Range, Type::Number),
                 (Type::Table(vec![]), Type::Record(vec![])),
                 (Type::Record(vec![]), Type::Record(vec![])),