Block_Info::read_block_costs(): use more realistic RAM estimate

[vasdommes]

Currently Schur block size is used as a block cost to balance RAM during a timing run.
See https://github.com/davidsd/sdpb/blob/6ad4c404e5a208a44c606598fdd5f3c7fea089b4/src/sdp_solve/Block_Info/read_block_costs.cxx#L49C1-L59C6:

      // If no information, assign a cost proportional to the matrix
      // size.  This should balance out memory use when doing a timing
      // run.
      auto schur_sizes(schur_block_sizes());
      for(size_t block = 0; block < schur_sizes.size(); ++block)
        {
          result.emplace_back(schur_sizes[block] * schur_sizes[block], block);
        }

In reality, RAM usage is something like 2#(B) + 5#(PSD) + 2#(Schur) + 2#(Bilinear pairing), usually dominated by B matrix (in code - free_var_matrix). We should use this estimate instead of currently used #(Schur).