incorporate different cve computation methods to compute_cve

Based on discussions in #87:
- We load everything from the `fast_cve` module to the `functions` module.
- Specifically, `compute_cve` should take `DO`, `muD`, and `method` as inputs and return a `cve DO`. It calls `cve_method` through the `method` and passes `DO` and `muD` to compute the cve based on the `global_tth`. We then interpolate the cve to the original grid.