ENH: Added new limitFields function object

@Roger @Prashant - can you confirm this OK to go in? and any test case/updated tutorial that can be provided?

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Details of the new function object:

Limits fields to user-specified min and max bounds

Usage Example of function object specification:

limitFields1
{
    type        limitFields;
    libs        ("libfieldFunctionObjects.so");
    ...
    fields      (U);
    limit       max;
    max         100;
}

Where the entries comprise: Property | Description | Required | Default value type | type name: limitFields | yes | fields | list of fields to process | yes | limit | bound to limit - see below | yes |

The limit entry can take the value:

• min : specify a minimum value
• max : specify a maximum value
• both : specify a minimum value and a maximum value