LA: add a fast path for updating matrix values

[andrea-iob]

The fast path will use the PETSc function MatSetValuesRow to update all the values of a row at once (without the need to get the row pattern).

Fast update can be performed if:

• the matrix has already been assembled;
• the assembler is providing all the values of the row;
• values provided by the assembler are sorted by ascending column.