Updated tutorial for transient PDEs

[AlexandreMagueresse]

The tutorials needed to be updated following the refactoring of the ODE module of Gridap. I have edited the previous tutorial on the heat equation and added a new tutorial on a nonlinear variant of the heat equation (where the diffusivity coefficient is polynomial in u) to illustrate the difference in the description of the ODE operator.

[JordiManyer]

@amartinhuertas Could we approve this?

[amartinhuertas]

@amartinhuertas Could we approve this?

Is the PR ready to be merged?

[AlexandreMagueresse]

The versions of Gridap and GridapMakie may need to be checked or updated in the project file. (Last time I checked, all the tutorials were compatible with the last version of GridapMakie). The rest is ready to be merged.

[JordiManyer]

Can we merge this now that GridapMakie has been updated?

[amartinhuertas]

Yes. Accepting ...