Implementation of FFTW and covariance matrix functionality added

Se_inv: is now handled differently, defaults to eye-matrix, but can be set
        later (added a few convenience functions to DataSet)
utility: (re-)added utility module for index conversion
kernel/phasemapper: can now use FFTW library and operate on pre-allocated
        arrays
dtype: set dtype of arrays to np.float32/np.complex64 (halves memory size),
       also applied to numcore routines!
magdata: quiver_plot can now plot every n-th arrow (yet untested!)
reconstruction scripts: modified several reconstruction scripts.