Also added missing 'const' to the getMemoryElement method for host.

[skip ci]

Refactoring: replaced TNL::SharedPointer with std::shared_ptr for the linear solver and preconditioner

The DeviceType capability does not matter because these objects are
never passed to the device and we need to pass "shared_ptr<Derived>" as
"shared_ptr<Base>".

They are virtual, currently not needed and only complicate things.

The vector view types have to be passed by value, because pass by
reference does not consider the converting constructors.

The x and b vectors might have different type.

It is actually used only in 2 methods and unnecessarily complicates the
interface for all other solvers.

This reverts commit b9b95d10.