New mesh readers/writers, Grid and DistributedGrid refactoring

Changelog:

• fixed various issues with mesh readers and writers
• added tests for mesh readers and sample data
• implemented VTIWriter and VTIReader, removed TNLReader
• removed methods save and load from Grid, Mesh, MeshFunction, MeshFunctionView and VectorField
• implemented MeshFunctionIO: reading and writing mesh functions using mesh readers and writers
• tools, examples and benchmarks were updated to use functions from MeshFunctionIO
• removed tnl-lattice-init - it was intended only for the tnl-lbm project which is deprecated (did not lead anywhere)
• removed tnl-view - it has become obsolete, since all mesh functions are going to be saved directly in the VTK/VTU/VTI format and thus the conversion from .tnl files is not necessary
• refucktored DistributedGrid
• implemented PVTIWriter and PVTIReader, extended MeshFunctionIO for distributed grids
• removed DistributedGridIO - it was replaced with PVTIWriter and PVTIReader