Remove unnecessary check of chart.model

When dealing with pieChart, there is no axislist. Hence,
we are ok, when we don't check this.
Remove complex if else,

We can simply set the value of chart.show_legend by using a
jinja filter
Remove more pieChart specific stuff from content
Remove closing and opening from base.html

This is already handled by content.html
Don't call super in pie.html
Remove closing script from content

Add infra template. This template eventually should replace
content, but it should be much more sparse, since most
of the if else structures will go to the individual graph
templates.
Fix a bug introduced earlier in scatterChart
Remove scatterChart only attributes from content

Add them to scatterChart template
Add data block

In this block the data series of the graph is written
Line chart template now extends content
Discrete bar chart has it's own template now
Use new style class for NVD3Chart

This allows calling super
Add a new class DiscreteBarChart

Note the capital D. This is a new class which does not
render so many templates like discreteBarChart. Rather,
this class renders a templates which exploits Jinja
template inheritance. This template simply extends the template
content.html.

Future release can do:

    discreteBarChart = DiscreteBarChart

Such that, if discreteBarChart is removed the old class is still
available from the user's point of view.

Also, this class violates the DRY principle, it overrides methods
of NVD3Chart base class. This pattern will be introduced by all
other Capital letter new classes. This methods can be moved later to
a Mixin class.
Add LineChart which uses template inheritance
Add LinePlusBarWithFocusChart which supports inheritance
Add CumulativeLineChart
Add LinePlusBarChart
Add LineWithFocusChart
Add MultiBarChart
Add MultiBarHorizontalChart