Create a new function for diffusivity

[phydev]

The calculation of the diffusion coefficient should be moved to a new @staticmethod (see bellow).
https://github.com/phydev/trajpy/blob/562415b0f35976c726df7fc8daa773dacca7fb2c/trajpy/trajpy.py#L74

@staticmethod
def diffusivity_(msd, timelag, ndim):
    """"
        :param msd: ensemble averaged mean squared displacement
        :param timelag: time-lag
        :param ndim: number of dimensions
        :return diffusivity: short-time diffusion coefficient D 
    """"
    diffusivity = Trajectory.anomalous_exponent_(msd[:10], timelag[:10]) / (2*n)
    return diffusivity

[phydev]

Diffusivity should be generalized with Green-Kubo relation.

[phydev]

Commit 4e620cb: added three new functions to the class Trajectory in order to compute the Green-Kubo relation:

• velocity: computes the instantaneous velocity along the trajectory
• _stationary_velocity_correlation: estimate the stationary velocity correlation
• green_kubo_: calculates the diffusivity by integrating the stationary velocity correlation

A new test covering the new functions was added to tests/computing_features.py