add band structure calculation

[xuqimen]

• Add band structure calculation feature.
• Add readDens_cube routine for reading input density file.
• Fix memory leak issues:
  • when ixc[2] is nonzero, countPotentialCalculate is not initialized,
  • EigenFilename is not initialized on processes with rank > 0.
• Add a python script in utils/ for processing .eigen files and visualizing the band structure of a given system.