Plex: Update examples to use new creation interface

Merge branch 'knepley/feature-swarm-fortran' into 'master'

Knepley/feature swarm fortran

See merge request petsc/petsc!2680

Swarm ex1f: Added tutorial example for projection
- Can visualize particles and particle fields through Xdmf
- fixed to conserve, added print of norm
- Added C Swarm ex1 with with indexing for XGC

C

Swarm ex1f: Added tutorial example for projection
- Can visualize particles and particle fields through Xdmf
- fixed to conserve, added print of norm
- Added C Swarm ex1 with with indexing for XGC

Co-authored-by: Mark Adams <mfadams@lbl.gov>

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