mesh_maker needs to calculate correct mask (i.e. not 1 everywhere)

### Brief summary of bug

I used mesh_maker to generate the ndep stream mesh for TRENDY2026, and we found that it didn't work because the mask was set to 1 everywhere.

@swensosc used his script to generate a mesh that worked. Sean's script calculates the correct mask using the fill values from the input file (e.g. ndep stream in this case).

Definition of DONE:
- [ ] Bring Sean's mask algorithm or similar into the mesh_maker tool.

### General bug information

**CTSM version you are using:** ctsm5.4...

**Does this bug cause significantly incorrect results in the model's science?** No, but mesh_maker has limited use.

**Configurations affected:** Making mesh files for streams.

### Details of bug

Interim solutions:
- Run Sean's script
- Possibly run mesh_maker followed by mesh_mask_modifier...