Triqs: GreenDLR Basic

[fsxbhyy]

-Complete the TODO list for basic functions in GreenDLR, including:

• [x] 1.new functions:
  • [x] Basic.show for nice print, related to issue #8
  • [ ] Basic << for initialization and assign value
  • [x] density for calculating the density from Green'sfunction
• [x] 2. Assertion and edge cases: Add assertions to make sure the user input is eligible for functions
  • [x] 3. Unit test Add test in test_GreenDLR.jl