Transform cell-based to face-baced connectivity in solver instead of preprocessor

Currently, the cell->vertices to face->cells and face->vertices connectivity conversion is done in the proprocessor. Migrating this to the solver requires modifying the preprocessor reader, but would have several advantages:

- Lower memory usage (about half) in the non-parallel preprocessor
- Migrate all postprocessing and mesh checking aspects to the solver
- prerequisite to allow adding parallel mesh readers directly in solver

This could also be done in a separate order, adding specific readers to the solver (reading into an FVM mesh for formats such as MED or CGNS, allowing interpolation operations) first.

Note: this is an update from an issue on the old (Mantis) issue tracker, which is a long-term goal.

まず、preprocessor reader の cell-to-vertex、face-to-cell、face-to-vertex の接続性変換を追跡し、次に solver の FVM mesh エントリポイントを調査します。既存の reader のサポートを維持し、将来の並列 reader を可能にしたうえで、接続性変換、postprocessing、mesh checking が solver に移行されれば、作業は完了です。

索引モデルが issue の本文から書いたものです。