Change tests for dataclasses (StructureData, TrajectoryData) to call the
renamed functions .get_structures() (former ._get_aiida_structures()) and
.get_cif() (former ._get_cif())

Change TrajectoryData to make passing stepids and cells optional. While
nothing will be stored for cells if not given a consecutive sequence will be
automatically assigned to stepids if missing from the inputs.

As proposed in issue aiidateam#201 symbols are now stored in a
TrajectoryData attribute rather than an array to simplify the
query for these symbols

…ls array

Move set of pylint options inside CifData class. Disable check for
too-many-public-methods complaining about 21/20 public methods
after renaming _get_aiida_structure -> get_structure making get_structure()
method public

Make class methods .is_alloy() and .has_vacancies() for StrutureData
and Kind classes accessible as class properties .is_alloy and
.has_vacancies

In function _internal_validate the 'symbols' list containing
atomic names is checked for being 1D using the shape of the
array. However, each entry of this list is ensured to be of type
six.string_types somewhere upstream in this function which would
already have triggered if the list is not purely 1D (contains one
or more lists as entry somewhere)

Had to reformat the warning.warn() function arguments to get pylint
invoke the set 'disable=no-member' option.