Added a parser for OFFSETS and CONNECTIVITY tags within legacy vtk files
that mimic the way the xml file work.

There is a new version of legacy vtk files (seemingly v5 and up) that
specify cells via OFFSETS and CONNECTIVITY tags. This parser adds
support for that.

See issue #21 for exmaples.

This commit addresses the parsing of the legacy files in that issue.

Added the ability to write cells in new offsets + connectivity format
for vtk versions 5+.

See issue #21 for details.

Usage of this function is replaced by the generic write_vec function
implemented in the previous commit.

This is where prototype assets can be stored which are used for
temporary testing or large files that should not be committed to the
repo.

This is taken from issue #21.

All credit for these test files and gen.py script goes to @2pt0

Since some files can give non uint8 type cell types, we explicitly cast
everything to the right type instead of expecting it to be given
correctly. This helps parse files generated by meshio (See issue #21)

One of the previous commits fixes the issue. This commit fixes the test for
meshio generated files.

Clarify usage of xml and legacy formats, without renaming (yet).

This makes the results more consistent with paraview.
It also passes on one additional testcase from pygmsh.

Each inline (non-appended) binary DataArray now supports compression in the
same way appended data did before.

This commit concludes fixes for all failing meshes from issue #21.

The remaining pygmsh tests are now enabled.