Mesh domain based constants

### Describe new/missing feature

In complex large engineering problems, there are several parameters in domain and boundary integrals. They are constants within sub-domain and boundary faces. The only way that can be handled at the moment is through coefficients. As the mesh size goes up, its hitting with memory issues, due to the storage requirement of several same values through coefficients.
To be more specific, these parameters are constants within regions of mesh (Meshtags for domain and boundary faces).
To address this issue, concept of constants for meshtags of domains and boundary faces will be useful.

### Suggested user interface

```python3

```

The issue does not identify files, tests, or implementation entry points; begin by locating the mesh-tag and coefficient APIs in the repository. Define the Python-facing interface for constants on domain and boundary meshtags, then verify that large meshes avoid storing repeated coefficient values and that domain and boundary integrals use the constants correctly.

Written by the indexing model from the issue text.