`covariance_i_j` computes the approximate covariance tensor of the functional data for lag windows defined by i,j.
covariance_i_j(f_data, i, j)
A 4-dimensional array, encoding the covariance tensor of the functional data for lag windows defined by i,j.
the functional data matrix with observed functions in the columns
the indices i,j in 1:T that we are computing the covariance for