ENH: Added new hydrostatic pressure function object

Calculates and outputs the pressure fields p_rgh and ph_rgh based on the
option that was previously hard-coded in the fireFoam solver

Usage
    Example of function object specification to calculate hydrostatic pressure:
    \verbatim
    hydrostaticPressure1
    {
        type        hydrostaticPressure;
        libs        ("libinitialisationFunctionObjects.so");
        ...
    }
    \endverbatim

    Where the entries comprise:
    \table
        Property     | Description             | Required    | Default value
        log          | Log to standard output  | no          | yes
        p_rgh        | Name of p_rgh field     | no          | p_rgh
        ph_rgh       | Name of ph_rgh field    | no          | ph_rgh
        pRef         | Name of pressure ref field | no       | pRef
        rho          | Name of density field   | no          | rho
        U            | Name of velocity field  | no          | U
        gh           | Name of gravity*height volume field  | no | gh
        ghf          | Name of gravity*height surface field | no | ghf
        nCorrectors  | Number of correctors when solving ph_rgh | no | 5
    \endtable

Note
    Calculates the hydrostatic pressure on construction/re-initialisation;
    the execute and write functions are not used.