dictionary: Set the default scoping syntax to 'slash'

The new optional 'slash' scoping syntax is now the default and provides a more
intuitive and flexible syntax than the previous 'dot' syntax, corresponding to
the common directory/file access syntax used in UNIX, providing support for
reading entries from other dictionary files.

In the 'slash' syntax
    '/' is the scope operator
    '../' is the parent dictionary scope operator
    '!' is the top-level dictionary scope operator

Examples:

    internalField 3.4;

    active
    {
        type            fixedValue;
        value.air       $internalField;
    }

    inactive
    {
        type            anotherFixedValue;

        value           $../active/value.air;
        anotherValue    $!active/value.air;

        sub
        {
            value           $../../active/value.air;
            anotherValue    $!active/value.air;
        }
    }

    "U.*"
    {
        solver GAMG;
    }

    e.air
    {
        $U.air;
    }

    external
    {
        value $testSlashDict2!active/value.air;
    }

    active2
    {
        $testSlashDict2!active;
    }

If there is a part of the keyword before the '!' then this is taken to be the
file name of the dictionary from which the entry will be looked-up using the
part of the keyword after the '!'.  For example given a file testSlashDict containing

    internalField 5.6;

    active
    {
        type            fixedValue;
        value.air       $internalField;
    }

entries from it can be read directly from another file, e.g.

    external
    {
        value $testSlashDict2!active/value.air;
    }

    active2
    {
        $testSlashDict2!active;
    }

    which expands to

    external
    {
        value           5.6;
    }

    active2
    {
        type            fixedValue;
        value.air       5.6;
    }

These examples are provided in applications/test/dictionary.

The the default syntax can be changed from 'slash' to 'dot' in etc/controlDict
to revert to the previous behaviour:

OptimisationSwitches
{
.
.
.
    // Default dictionary scoping syntax
    inputSyntax slash;  // Change to dot for previous behaviour
}

or within a specific dictionary by adding the entry

See applications/test/dictionary/testDotDict.

src/OpenFOAM/db/dictionary/functionEntries/inputSyntaxEntry/inputSyntaxEntry.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     | Website:  https://openfoam.org
-    \\  /    A nd           | Copyright (C) 2019 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2019-2020 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -49,7 +49,7 @@ Foam::functionEntries::inputSyntaxEntry::inputSyntax
             "inputSyntax"
         )
     )
-  : DOT
+  : SLASH
 );
 
 // Initialise the current dictionary syntax to the default