GridGen: Soil types interpolation from planar cross-sections

New features:

• Soil types interpolation from planar cross-sections given in another file.
• New demonstration example (ex6 project).

Updates:

• Tecplot output file written in BLOCK data packing format to enable support of cell-centered data such as soil type ID.
• Update copyright notice in all files headers to be compliant with GPL v3 requirements.