Misc pending tasks associated with refactoring in branch release-0.2

## Merging into `master` PR https://github.com/gridap/GridapDistributed.jl/pull/50

- [x] Update README
- [x] remove src/OLD, test/OLD, compile
- [x] Add NEWS.md

## High priority
- [x] Release Gridap 0.17.0
- [x] Release PartitonedArrays 0.2.3
- [x] Write data into `pvtu` format (@amartinhuertas) PR https://github.com/gridap/WriteVTK.jl/pull/1 https://github.com/gridap/GridapDistributed.jl/pull/46
- [x] Create MPI launchers for the test (now we just test in serial mode) @fverdugo https://github.com/gridap/GridapDistributed.jl/pull/45
- [x] Assembly of the rhs vector alone (@amartinhuertas) PR https://github.com/gridap/GridapDistributed.jl/pull/40
- [x] Solve broken tests in `FESpacesTests.jl` namely `assemble_XXX!` functions do not work @fverdugo https://github.com/gridap/GridapDistributed.jl/pull/44
- [x] Implement assembly strategy when one also iterates over ghost cells.
- [x] Interpolation onto distributed multi-field spaces. @fverdugo PR https://github.com/gridap/GridapDistributed.jl/pull/42
- [x] Port uniformly refined forest of octrees distributed model this new version (in [GridapP4est](https://github.com/gridap/GridapP4est.jl)) @amartinhuertas
- [x] Add test drivers including DG examples. @fverdugo PR https://github.com/gridap/GridapDistributed.jl/pull/42
- [x] Add test drivers including nonlinear examples. @fverdugo PR https://github.com/gridap/GridapDistributed.jl/pull/44
- [x] https://github.com/fverdugo/PartitionedArrays.jl/issues/33 https://github.com/fverdugo/PartitionedArrays.jl/pull/35
- [x] `@assert`s locally triggered from the processors may lead to deadlock! We should call `mpi_abort` whenever an `AssertionError` exception is triggered. PR https://github.com/fverdugo/PartitionedArrays.jl/pull/39
- [ ] AD in parallel. Talk to @amartinhuertas to grasp the challenges there.

## Medium priority
- [x] Nicer user API for PETScSolver (to be done in GridapPETSc). In particular, better handling of the lifetime of PETSc objects.
- [x] Implement distributed models from gmsh (in GridapGmsh) @fverdugo
- [x] implement `lu!` for PSparseMatrix following the same strategy as with \
- [x] Move mpi tests to their own CI job @amartinhuertas PR https://github.com/gridap/GridapDistributed.jl/pull/47
- [x] Automatically generate sysimage to reduce mpi tests runtime @amartinhuertas PR https://github.com/gridap/GridapDistributed.jl/pull/48
- [ ] Think how the user can define the local and global vector type in the FESpace
- [ ] Showing `NLSolve.jl` solver trace only in master MPI rank leads to a dead lock.
- [ ] Peridodic BCs
- [ ] Implement ZeroMeanFESpace

## Low priority
- [ ] Implement a more lazy initialization of the matrix exchanger in PartitionedArrasys since the exchanger is not always needed.
- [x] Overlap compression of the sparse matrix with communication of rhs assembly
- [ ] Implement another strategy to represent local matrices in PartitionedArrays with a data layout compatible with petsc
- [x] `interpolate_everywhere` not available in `GridapDistributed.jl`, only `interpolate` Solved in PR https://github.com/gridap/GridapDistributed.jl/pull/74