[Field] refactoring and update

[MuellerSeb]

Overall

• drop usage of pos2xyz and xyz2pos

Generators

• remove structured option from generators (structured pos need to be converted first)
• explicitly assert dim=2,3 when generating vector fields

Tools

• remove routines pos2xyz and xyz2pos (finally)
• add routines to format struct. pos tuple by given dim or shape
• add routine to format struct. pos tuple by given shape (variogram helper)
• remove field.tools subpackage

Field

• simplify pre_pos routine to save pos tuple and reformat it an unstructured tuple
• simplify field shaping
• simplify plotting routines

SRF/Krige

• simplify generation and conditioning

Mesh

• support meshio>=4.0
• add meshio as dependency
• add example

[MuellerSeb]

@LSchueler should be ready for a first glimpse.

[MuellerSeb]

@LSchueler there is still a bug in format_unstruct_pos_shape that needs to be tackled by me. Just to keep that in mind.

Edit: Solved.

[LSchueler]

That's already looking so great!

As always, I've fixed some typos, so remember to pull before doing new commits.

I guess the gstools.field.summator.summate_{unstruct,summate_incompr_unstruct} should be renamed. The _unstruct is probably only confusing from now on. Or what reasons are there to keep the name? The name could be summate_modes if you don't like summator.summate.

Apart from that, I don't have too much to add at the moment, except maybe: great job!