LA: add a function to display information about a sparse matrix

[andrea-iob]

This is an example of the information that will be displayed:

General information
  Block size ............................................ 5
Local information: 
  Maximum number of non-zero blocks per row ............. 25
  Maximum number of non-zero elements per row ........... 125
  Number of block rows .................................. 1296
  Number of block columns ............................... 1296
  Number of non-zero blocks (pattern) ................... 28781
  Number of non-zero elements (pattern) ................. 719525
  Number of non-zero blocks (non-neglibile values) ...... 28737
  Number of non-zero elements (non-neglibile values) .... 393235
  Sparsity of the blocks (pattern) ...................... 0.982865
  Sparsity of the elements (pattern) .................... 0.982865
  Sparsity of the blocks (non-neglibile values) ......... 0.982891
  Sparsity of the elements (non-neglibile values) ....... 0.990635
  Memory used by the value storage ...................... 5.48954 MB
Global information: 
  Maximum number of non-zero blocks per row ............. 25
  Maximum number of non-zero elements per row ........... 125
  Number of block columns ............................... 3888
  Number of block rows .................................. 3888
  Number of non-zero blocks (pattern) ................... 85128
  Number of non-zero elements (pattern) ................. 2128200
  Number of non-zero blocks (non-neglibile values) ...... 84958
  Number of non-zero elements (non-neglibile values) .... 1174396
  Sparsity of the blocks (pattern) ...................... 0.994369
  Sparsity of the elements (pattern) .................... 0.994369
  Sparsity of the blocks (non-neglibile values) ......... 0.99438
  Sparsity of the elements (non-neglibile values) ....... 0.996892
  Memory used by the value storage ...................... 16.2369 MB