Feature dictionary scoping lvalue

• Supports using '/' within dictionary scoping, which avoids the ambiguity of having a '.' which could mean scoping, or be part of a keyword.
• General dictionary searcher classes/methods for clearer searching semantics and to enable addressing a dictionary entry as an lvalue.
• The #remove directive now functions across scopes
• Can specify a keyword with a defined scope. If needed, any intermediate dictionaries are created.

For example,

"/boundaryField/lowerWall/type"  noSlip;
"/boundaryField/newBoundary"  { type fixedValue; value uniform (0 0 0); }

Manage "one-shot" change of the inputMode. For example,

#includeIfPresent "user-files"
...
#default value uniform 10;

vs.

#includeIfPresent "user-files"
#inputMode protect
...
value uniform 10;
#inputMode merge    // _Assuming_ we actually had this before