v0.6.4

Keldysh v0.6.4

Diff since v0.6.3

Closed issues:

• No arithmetic operations for GF objects (#10)

Merged pull requests:

• [dos] Add spectrum shift argument to bethe_dos(), chain_dos() and square_dos() (#14) (@krivenko)
• Basic arithmetic operations for all GF containers (fix #10) (#15) (@krivenko)