Add a new filtering alteration to drop rows where some or all columns has a null value

[Seddryck]

When parsing some weird files, it would be convenient to be able to drop rows where at least one column has the value null. On some other cases, it's when all the columns are null that the row should be dropped. Writing this with the traditional filter can become cumbersome when there are a lot of columns or you don't know how many columns your file will contain.

<alteration>
  <drop>
    <some-columns> // all-columns
      <null />
    <some-columns>
  </drop> 
</alteration>