Feature/utils coverage

These are pugs initialize and finalize functions.

These functions cannot be checked in the unit-tests framework.

Also rename test file (getting rid of the 'mpi_' prefix)

Actually, it is not easy to check these utilities and could be risky.
Checking these features could change the behavior of the other tests
unexpectedly.

Actually fixed a few issues and make functions escapeString() and
unescapeString() reciprocal.

This later change leads to a grammar change, now literal do not
store quotes: before `"foo"` was stored as the string "\"foo\"", now
it is just stored as "foo".

This prevent some weird implicit constructions that could occur
chaining implicit conversions

Also made a few cosmetic cleanups

The considered code was an alternate strategy to define cell-cell
connectivity: through nodes.

Also add a bunch of missing [[nodiscard]] qualifiers and fixed its
effects.