Transpose Linop

[yhmtsai]

The idea is to add transpose as linop to allow non-explicit construction of transposition.
The current IC will return one Linop if do not construct the transposed matrix.
With transpose Linop, we can always return two linop in this situation.

Usage:
Transpose(LinOp* , explicit=false): constructor
construct(): construct the explicit transposition
apply(): if the transposition is not built,

1. always needs one transpose before apply
2. store the transposition and perform apply (need mutable to change it in const function)
3. If the operation has transpose apply, directly use it (no linop implement transpose_apply)

Putting IC into preconditioner needs to convert the composition to csr.
For converting on transpose LinOp, we may always do explicit transpose and perform the conversion