Dev: Removing row and column .cells() methods

- Now that we have the cells() / cell() methods, with the ability to
  select based on column and row, and the ability to get the whole row
  or column cells, the TD get methods on row / rows / columns were
  redundant and I think should be removed for berevity. It could also
  get confusing with the chaining since there are nested and top level
  methods with the same name.

- Removed:
  - rows().cells()
  - row().cells()
  - columns().cells()
API: Consistency of data return for columns().data()

- The columns().data() method was flattening the returned array. While
  possibly okay on its own, that doesn't match the behaviour of how
  rows().data() works, where each row is an entry in the array, rather
  than the array being flattened. As such I've changed columns().data()
  to work work in the say way, for each column you select, you get an
  entry in the api instance - an array with the data for the that
  column.

- To allow for hte fact that some may want to work with the flattened
  data, I've added a `flatten()` method to the instance base, which is
  basically a shortcut call to `reduce`.