Dev tutorials

[ericneiva]

Hi, @santiagobadia,

I have gone through the poisson_dev_fe tutorial and applied some changes.

I have a couple of questions:

1. L234: does not involve allocations?
2. What is L400 for?

I have already used this tutorial for my own devs. Thanks for preparing it!

Please, review and accept this merge request whenever you have some time. Regards.

[ericneiva]

I forgot to mention one thing:

L198 I found a little bit confusing that you mentioned the @show withouth invoking it... Maybe the sentence could start with If you use the @show method...