Initial Support for Sparse Direct Solvers

[youwuyou]

With a small refactoring of the LinOp interface, where we define the GkoLinOp as an abstract type with GkoIterativeSolver and GkoDirectSolver solvers as concrete data type, it would enable more comprehensive solver initialization and destructor using the same linop delete.

refactor interface

• change LinOp.jl and export types accordingly in Ginkgo.jl file
• change previous examples where GkoLinOp was involved
• add tests for solvers

add examples

• stokes continuity
• heat conservation
• slab breakoff