The std::isnan method can be safely used in host code (if the compiler
is not moron). There is no device code which uses isnan so the
TNL::isNan function is useless.

This reverts commit 0eff2f63.

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>".

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

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

They are disabled and never really worked for these objects...