Sponge zone added

[marc-85]

A Sponge Zone capability has been added. The definition of the portion of the domain where the sponge correction is applied follows the same approach as that followed in the linearly varying viscosity (some info can be read in dataStructures.hpp).
Two functioning modes have been implemented:

• User-defined target solution. When using this option the user is required to provide the target primitive values. This option can be defined in the runfile as follows:

SZ_PLANE_NORM -1 0 0
SZ_PLANE_P0 8 0 0
SZ_PLANE_PINIT 5 0 0
SZ_TYPE 0 1.2 55 0 101300
SZ_MULT 2.5

• Mixed-Out approach. This mode does not require for the user to input a target solution. Instead, this is computed as the mixed-out solution at the initial plane. Since this method uses nodal data to compute the mixed-out values, this options requires of an additional value which represents the distance tolerance that will be used to find the nodes, i.e., nodes which are within +-dist of the initial plane will be used to compute mixed-out values.This is handy when the solution is not known a priori, as it is the case in internal flows. The previous example can be made to use mixed-out variables as

SZ_PLANE_NORM -1 0 0
SZ_PLANE_P0 8 0 0
SZ_PLANE_PINIT 5 0 0
SZ_TYPE 1 1e-1
SZ_MULT 2.5

The intensity at which the target field is imposed depends on the averaged speed of sound and the length of the sponge zone. This, however, can be modified with the option SZ_MULT. Values larger than 1 will impose the target values faster than the default; values lower than 1 will result in a weaker sponge zone; and the value of 1 is the default.

This PR addresses issue #79