Elastic properties

[Mellechowicz]

From TODO list:
A new module elastic for calculation of the elastic properties
from the stress-strain relation based on VASP's vasprun.xml file.

Module elastic.CMatrix contains a class ElasticTensor
with all 9 different symmetry-based constrains 6x6 elastic tensors.

Submodule elastic.elastic contains two classes:
(i) Calculation containing the scipy-based engine,
fitting the elastic tensor C to the stress-strain relation.
(ii) Properites with the methods deriving the elastic properties
from the elastic tensor (within different approximations)

Module elastic.readdata contains a class XMLParser extracting
both stress and strain from the vasprun.xml

Additionally scripts/vasprun contains 3 new flags

  -E, --elastic         run calculations for the elastic properties (requires
                        IBRION >=5), default: false
  -C SYMMETRY, --elasticsymmetry=SYMMETRY
                        symmetry group of the supercell, default: Triclinic;
                        For calculation of elastic properties only!
                        choices: "Triclinic", "Monoclinic", "Orthorhombic",
                        "Trigonal6", "Trigonal7", "Hexagonal",
                        "Tetragonal6", "Tetragonal7", and "Cubic"
  -A APPROXIMATION, --approximation=APPROXIMATION
                        approximation used for the calculation of the elastic
                        moduli, default: voigt;
                        For calculation of elastic properties only!
                        choices: "voigt", "reuss", "hill", and "all"

enabling this new functionality.

TODO: An automatic scheme of determining the symmetry group directly from vasprun.xml.