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.

首先跟踪预处理器 reader 的 cell-to-vertex、face-to-cell 和 face-to-vertex 连通性转换，然后检查 solver 的 FVM 网格入口点。当连通性转换、后处理和网格检查迁移到 solver，同时保留对现有 reader 的支持并支持未来的并行 reader 时，工作即告完成。

由索引模型根据 Issue 内容生成。