Make usage of valueAverage function object clearer / improve documentation / extend tutorials

Functionality to add/problem to solve

The usage of the value average function object is shown exemplarily for the forceCoeffs function object, which has a quite clear naming of the resulting variables. When usage with different function objects is intended, the user has to figure out the "resultName" from the source code of the desired function object.

Target audience

Any user that wishes to apply the valueAverage function object easily and quickly. Any cases that benefit from the valueAverage function object.

Proposal

Adding examples where the valueAverage function object is used in conjuction with different function objects than forceCoeffs. For example:

 functions
 {
     uni
     {
         type            surfaceFieldValue;
         libs            (fieldFunctionObjects);
         writeControl    timeStep;
         writeInterval   1;
         log             yes;
         writeTotalArea  yes;
         writeFields     no;
         regionType      patch;
         name            outlet;
         operation       uniformity;
         fields          (U);
     }
     valueAverage1
     {
         type        valueAverage;
         libs        ("libfieldFunctionObjects.so");
         writeToFile yes;
         log         yes;
         functionObject uni;
         fields      ("uniformity(outlet,U)");
         window      20;
     }
 }

What does success look like, and how can we measure that?

The completion of extended tutorials, happy users.

Funding

The functionality already exists for all relevant function objects, if not all. If you want me to come up with some regular use cases worthy of being added to tutorials, I will happily work out some examples and provide them.