functionObjectList: Moved the functions entry from controlDict into a functions file

for consistency with fvModels and fvConstraints, to simplify code and case
maintenance and to avoid the potentially complex functions entries being
unnecessarily parsed by utilities for which functionObject evaluation is
disabled.

The functions entry in controlDict is still read if the functions file is not
present for backward-compatibility, but it is advisable to migrate cases to use
the new functions file.

etc/caseDicts/functions/fields/age

@@ -0,0 +1,36 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     | Website:  https://openfoam.org
+    \\  /    A nd           | Version:  dev
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+Description
+    Solves a transport equation to determine the time taken for a particle to
+    convect from an inlet to the location in the flow.
+
+    This will output a field, age, with units of time. This field needs a
+    solver setting in fvSolution and a div(phi,age) scheme in fvSchemes.
+    Alternatively, a schemesField entry can be used to reuse settings from
+    another field.
+
+    The number of correctors, nCorr, determines how many times the solution is
+    repeated to iterate away any non-linearities in the choice of scheme. If
+    the divergence scheme is set to upwind, no corrections will be necessary.
+
+\*---------------------------------------------------------------------------*/
+
+type            age;
+libs            ("libfieldFunctionObjects.so");
+
+phi             phi;
+rho             rho;
+nCorr           5;
+schemesField    age;
+diffusion       false;
+tolerance       1e-5;
+
+executeControl  writeTime;
+writeControl    writeTime;
+
+// ************************************************************************* //