Allow no bndry

[dschwoerer]

fix #477 by fixing #376

Now, if the domain has no guards, and only one cell, the derivative just returns 0.

This will allow 2d / 1d simulations using 3d fields.

If this is the way we want to go, than making sure boundaries aren't set, and also brackets aren't evaluated, would allow to run e.g. 1D simulations, without changing the equations, but just setting NY=1 and MYG=0 ...

[dschwoerer]

Should I move the getters to field? (#498)
Or do you think this is the wrong way to approach this issue?

[JensMadsen]

I do not understand how the private boutmesh variables nx, ny and nz are in the scope of the fields?

[dschwoerer]

Because they are also private members of the fields:
https://github.com/boutproject/BOUT-dev/blob/master/include/field3d.hxx#L491
The comment is however a bit confusing, as they need to be valid, if the field has data:
https://github.com/boutproject/BOUT-dev/blob/master/include/field3d.hxx#L346

Also the loop operator uses these values:
https://github.com/boutproject/BOUT-dev/blob/master/src/field/field3d.cxx#L237

Further it makes sense that the field knows these values, as it is needs to know them, as otherwise it wouldn't be able to know the size of the data.

Note that nx etc. are not necessarily the same as for the mesh.
Field2D has e.g. always nz=1