New graph visualization recommendation for relational data

[chenluli]

In AVA v3.0, the graph visualization ability is unavailable, for we plan to refactor this module and use G2 spec to draw graph rather than G6 in v3.1. This is because we hope in AVA the relational data visualizations have same descriptive schema with statistic charts, so that they can be used together and share some common rules for advisor.
在 AVA v3.0 中，关系型数据的可视化能力尚未透出。在 v3.1 中，会重写这部分，并将关系图推荐结果使用 G2 spec 绘制。这是因为我们希望 AVA v3 中，关系型图表和统计图表底层可以用一套描述性语法，以便能够共用部分视觉通道的推荐能力，并更好地将统计图表和关系型图表结合使用。

Following todos about graph visualization in v2 will be handled in v3.1 :
v2 中关系图推荐模块的 todos 会统一在 v3.1 图推荐模块重构时解决：
#304 #303 #302 #301 #299