v0.2.0

ProblemReductions v0.2.0

Diff since v0.1.1

Merged pull requests:

• Friendly error people when the type signature is not correct (#113) (@GiggleLiu)
• Polish documents (#114) (@GiggleLiu)
• Refactor 2.0 (#116) (@GiggleLiu)
• Refactor interface for generic tensor networks (#117) (@GiggleLiu)