Creating staggered operators

[halpernf]

Starting from the original master branch, I'm creating forward and backward staggered operators. Well underway.

The following tasks are necessary:

• [x] Change original forward staggered operators grad, curl, div to centered finite differences
• [x] Create forward staggered operators, grad+, curl+, div+
• [x] Create backward staggered operators, grad-, curl-, div-
• [x] Express scalar and vector laplacians with correct combination of staggered operators
• [ ] Introduce interpolations where necessary, maybe as an option (FV vs Shashkov's mimetic approach use different dual spaces)
• [x] Create tests checking div( curl ) = 0 and curl( grad ) = 0 with the correct staggerings
• [x] Create accuracy order tests for a simple analytical solution