add material failure criteria

[akaszynski]

Resolve #109 by reading in failure criteria:

>>> from ansys.mapdl.reader import read_binary
>>> rst = read_binary('file.rst')
>>> rst.materials[1]['stress_failure_criteria']
{'XTEN': 0.5,
 'XCMP': -0.25,
 'YTEN': 0.5,
 'YCMP': -0.25,
 'ZTEN': 5.0,
 'ZCMP': -5.0,
 'XY': 0.5,
 'YZ': 3.0,
 'XZ': 3.0,
 'XYCP': 0.0,
 'YZCP': 0.0,
 'XZCP': 0.0,
 'XZIT': 0.0,
 'XZIC': 0.0,
 'YZIT': 0.0,
 'YZIC': 0.0}

Documentation for Result.materials now has:

        Materials may contain the key ``"stress_failure_criteria"``, which
        contains failure criteria information for temperature-dependent stress
        limits. This includes the following keys:

        - XTEN : Allowable tensile stress or strain in the x-direction. (Must
          be positive.)

        - XCMP : Allowable compressive stress or strain in the
          x-direction. (Defaults to negative of XTEN.)

        - YTEN : Allowable tensile stress or strain in the y-direction. (Must
          be positive.)

        - YCMP : Allowable compressive stress or strain in the
          y-direction. (Defaults to negative of YTEN.)

        - ZTEN : Allowable tensile stress or strain in the z-direction. (Must
          be positive.)

        - ZCMP : Allowable compressive stress or strain in the
          z-direction. (Defaults to negative of ZTEN.)

        - XY : Allowable XY stress or shear strain. (Must be positive.)

        - YZ : Allowable YZ stress or shear strain. (Must be positive.)

        - XZ : Allowable XZ stress or shear strain. (Must be positive.)

        - XYCP : XY coupling coefficient (Used only if Lab1 = S). Defaults to -1.0. [1]

        - YZCP : YZ coupling coefficient (Used only if Lab1 = S). Defaults to -1.0. [1]

        - XZCP : XZ coupling coefficient (Used only if Lab1 = S). Defaults to -1.0. [1]

        - XZIT : XZ tensile inclination parameter for Puck failure index (default =
          0.0)

        - XZIC : XZ compressive inclination parameter for Puck failure index
          (default = 0.0)

        - YZIT : YZ tensile inclination parameter for Puck failure index
          (default = 0.0)

        - YZIC : YZ compressive inclination parameter for Puck failure index
          (default = 0.0)