Added getters for various data members and a wrapper hint.

I exposed several data members through getters in order
to be able to develop custom readers in Python. Also added a
hint to export GetUpdateExtent() to Python. In this particular
case, I am combining the XML reader and the MPIImageReader
to read a structured grid. The XML reads the meta data, the
MPIImageReader reads the appended data using MPI/IO.

Change-Id: I7877ecc68f8b200f56ea258eca3bf871feccaaf5
Methods to compute and return unique array values.

Determining unique values is the "abstract" (or categorical)
equivalent of determining the range of interval-valued array.
It only makes sense for arrays with a small number of distinct
values, and this also happens to be when it is computationally
efficient to compute.

This adds methods `GetUniqueComponentValues` and
`UpdateDiscreteValueSet` to `vtkAbstractArray`.
The `GetUniqueComponentValues` method populates a `vtkVariantArray`
you pass with a list of unique values taken on by a component, or,
clears it to an empty array if there are more than
`vtkAbstractArray::MAX_DISCRETE_VALUES` distinct values present.

The unique values are identified by sampling at most log2(n)
tuples for large arrays, so obtaining the summary information --
which is cached once computed -- is an inexpensive operation.
This does mean that rare observations will go undetected.

You may also obtain a list of unique *tuples* instead of unique
component values by passing -1 for the component number.

Unique values are cached in the `vtkInformation` associated with the array.
This commit also adds 2 new key types: one for storing `vtkVariant`
objects as values, and another for storing vectors of variants.
A variant vector is used to store unique tuples and unique component values.

Change-Id: I241cf61372966bba381eb28d14bdc043a54ba197
Unit test new `vtkVariant` information key classes.

... and unit test the double and string information key
classes while we're at it.

Change-Id: I886973eda75bfe7d080e1f1446d4a6124e72c823
Improved well-needed documentation, clarified code that is too hard

Change-Id: Ia2c7259a904313ae5b6d0cb2258048f475037610
Fixed an incorrect assignment

Change-Id: Ieef1f9d45a10b50075da745e1ded0650784249ef
Initialize this->Axes in vtkChartXYZ.

This is required to prevent a segfault if SetGeometry has
not been called on a vtkChartXYZ.

Change-Id: I7dd972fb8d70c5eea88c84dd250e58422732e975
A single example case combining 2D and 3D

Change-Id: Ib9bd5c61c409efefe4bde51af16812a692ab452d
Make sure axis label are shown when looking at a 2D plane in 3D

Change-Id: Ie5e06e4f190ff5a354d863d1defd2b821059d04d
Merge topic '13469_fix_cube_axes_parallel_rendering' into master

907a59e7 Make sure axis label are shown when looking at a 2D plane in 3D
The HTG generator can now also generate random refinement patterns

Change-Id: Ie9012e7c20b0fe116012d3caa5417d1e5ddbbd51
Fixed formatting issues

Change-Id: I5e9c019d6f6ba385bafff2b4b3c74f9c83968afc
Merge topic 'ImproveHyperTrees' into master

07fd4fa2 Fixed formatting issues
4cb1169e The HTG generator can now also generate random refinement patterns
9c6465aa A single example case combining 2D and 3D
58fc8da5 Fixed an incorrect assignment
d42782c2 Improved well-needed documentation, clarified code that is too hard
Forgotten commits

Change-Id: Ic6a0ea46089ef2c187c87339c96711aa86f1c478
Restored functionality of regression tests

Change-Id: Iea47d732920f04dc53833e3d3eee4c3194e911db
Merge topic 'ImproveHyperTrees' into master

2f4d912c Restored functionality of regression tests
31c40238 Forgotten commits
Fixed PrintSelf defects

Change-Id: Ie0acc3c2e4e7e30a5ff20d603408edad8862896d
Fixed missing initialization of Output ivar (was causing errors)

Change-Id: I433e6fede4bc4f2bb5a5e46df01a51c6af94eefe
Revert "Fix log scale coloring"

This reverts commit 3fad9773d90d0b97508ec4ba030a0c54df71d710.

Change-Id: I209be2bf502d140d873ef1434f283869233e33a7
Merge topic '13082_log_scaling_issue_REVERT' into master

f8012427 Revert "Fix log scale coloring"
Merge topic 'FixHyperTreeGridSourceTclTests' into master

2f6deaa6 Fixed missing initialization of Output ivar (was causing errors)
cbf4fccf Fixed PrintSelf defects