<a name="Intro"></a>
<h1>Introduction</h1>
This example solves a 1D Poisson equation:

$
    u : [0, 1] \rightarrow R\\
    u'' = f\\
    u(0) = u0\\
    u(1) = u1
$

using a finite difference method on an equidistant grid with `K` discretization
points (`K` can be controlled with a command line parameter). The discretization
is done via the second order Taylor polynomial:

$
u(x + h) = u(x) - u'(x)h + 1/2 u''(x)h^2 + O(h^3)\\
u(x - h) = u(x) + u'(x)h + 1/2 u''(x)h^2 + O(h^3)  / +\\
---------------------- \\
-u(x - h) + 2u(x) + -u(x + h) = -f(x)h^2 + O(h^3)

$

For an equidistant grid with K "inner" discretization points $x1, ..., xk, $and
step size$ h = 1 / (K + 1)$, the formula produces a system of linear equations

$

           2u_1 - u_2     = -f_1 h^2 + u0\\
-u_(k-1) + 2u_k - u_(k+1) = -f_k h^2,       k = 2, ..., K - 1\\
-u_(K-1) + 2u_K           = -f_K h^2 + u1\\

$

which is then solved using Ginkgo's implementation of the CG method
preconditioned with block-Jacobi. It is also possible to specify on which
executor Ginkgo will solve the system via the command line.
The function $`f` $is set to $`f(x) = 6x`$ (making the solution $`u(x) = x^3`$), but
that can be changed in the `main` function.

The intention of the example is to show how Ginkgo can be integrated into
existing software - the `generate_stencil_matrix`, `generate_rhs`,
`print_solution`, `compute_error` and `main` function do not reference Ginkgo at
all (i.e. they could have been there before the application developer decided to
use Ginkgo, and the only part where Ginkgo is introduced is inside the
`solve_system` function.

<h3> About the example </h3>
