feat: add option for ignoring diag() when computing linkages

Miller & Blair (2009) points that: >There is some disagreement in the literature on whether the on-diagonal elements in A or L should be included or netted out of the summations (see, for example, Harrigan and McGilvray, 1988). To the extent that these “internal linkages” constitute part of Hirschman’s (1958, p. 100) “... input-provision, derived demand ... effects,” they are appropriately included. On the other hand, if one is specifically interested in a sector’s “backward dependence” on or linkage to the rest of the economy, they should be omitted. Default behavior is to include diagonal elements, but it should be up to the user.