Update force_constants.py

FC are now calculated not basing on the inverse displacement (matrix), multiplied by the force matrix. Instead, for every force-displacement tensor elements the force-displacement linear regression fit is computed. It is done in the simplified zero-interception algorithm, thus calculations are fast. Additionally, the fitting error is calculated.
Merge branch 'patch-1' of https://github.com/lebecki/phonopy into merge-patch-1
Implement option to switch fc fitting algorithms
Minor change