Change figure and axes output handling (#224)

* add `gridspec_kw` kwarg to `make_nd_plot` and forward all remaining kwargs to `plt.figure`

* change test for error raising to AttributeError _and_ TypeError, as different python versions seem to raise different errors

* stop returning the figure in the `samples.plot_nd` commands, adapt tests accordingly

* fix flake8 issues of incorrect indentation

* move axes setup into `AxesSeries` and `AxesDataFrame` and call this from `make_nd_axes`

* move bulk of axes setup into `AxesSeries` and `AxesDataFrame` initialisation, however still issues with slicing etc

* flake8 fixes

* fix some whitespace issues

* add missing docstrings

* change tests to test for `isinstance(axes, AxesDataFrame)` rather than `isinstance(axes, DataFrame)`

* reintroduce the constructor properties for `AxesDataFrame` that ensure slicing returns `AxesDataFrame` or `AxesSeries`, also some polishing

* simplify and remove some blank lines

* remove default `None` from `set_labels` methods

* add tests for `AxesSeries` and `AxesDataFrame`

* further parametrize `test_make_2d_axes_behaviour`

* turn `make_diagonal` into a staticmethod and `_axes_frame` into a classmethod

* add lower/diagonal/upper functionality to `axlines` and `axspans` such that you can for example set different truths for the lower and upper triangle

* fix minor ticks and change tick defaults

* fix issue with incorrect minor ticks
  (This was particularly annoying because their is a default value set
  in pandas `plotting.core` module.)
* add `tick_params` method to `AxesSeries` and `AxesDataFrame`
* change default tick labelrotation to 45
* change default tick labelsize to `'small'`

* change default tick behaviour to `"inner"`

* ensure desired kde limits are computed before sample compression

* add `set_margins` method to `AxesDataFrame` to allow changing/padding axes limits without having to specify precise limits for each parameter

* add test for `axes.set_margins` method

* add `scatter` method for `AxesDataFrame` which allows to plot truth values as crosses or points rather than vertical and horizontal lines

* change signature of `axlines` and `axspans` to take a dictionary of params, modify tests accordingly, and add test for `axes.scatter` method

* switch to `from pandas import Series, DataFrame`

* clean up `AxesSeries` initialisation a bit

* fix ylim range for `kde_1d` plots dependent on `density` kwarg

* fix flake8 issue of too long line

* attempt at fixing master merge

* use `items` instead of `iteritems`

* add the call to `make_offdiagonal` which got lost in the master merge

* only apply `tight_layout` if no `gridspec_kw` are passed, else it does not work anyhow and throws warnings