Change variable names to better match tex

* everywhere: fk is now f
* lpca_agg_data: the subtree used as the aggregation seed x is now b (for branch)
* lpca_agg_data: the list of sibling subtrees xs is now bs
* lpca_agg_data: an individiual sibling subtree xi is now bj
* lpca_tot_edge_nz: the centered f vector fk' is now f_cen
* whitespace normalization
Eliminate need for carrying around mk