Feature tangent

[sanathkeshav]

• Algorithmic tangent via finite differences : 2 new methods Solver::get_homogenized_stress() and Solver::get_homogenized_tangent(pert_param)
• New Linear elastic triclinic material model with 21 inputs -> all C
• Type of Reader::materialproperties changed from map<string, vector<double>> to json -> material_properties in input json can accept many more things other than just vectors of doubles
• Introduced new field in input json called error_parameters that can definite error measure (L1, L2, Linfinity) and type (absolute, relative)