Add linear theory covariance matrix to 5pt options

Created by: voisey

I have added Zahari's construction of the theoretical covariance matrix to the 5pt prescriptions and assigned it the label 'linear' for the fivetheories flag. Could someone please cross check the equations I have written with those in Zahari's note? Note that the way delta(-,0) and delta(0,-) are defined in the note are different to the way they are defined in Richard's and Rosalyn's notes and in validphys, so the equations will differ by some minuses.