Time stepping and new features

[pmartorell]

@principejavier

This PR includes the following new features:

• Time-stepping for MHD on top of Gridap.ODEs machinery (application: transient)
• Deactivate the convection terms at the driver level (through convection=false)
• Set the number of non-linear iterations at driver level (through niter)
• Set initial guess for non-linear iterations at driver level
• Adding a convective stabilization term a the weak form [ 10.1137/21M1416539 ]

It also adds a new application: channel. It is a square duct with an optional non-uniform magnetic field.