a constants module

[miniufo]

xinvert uses many constants within the package. However, these constants need to be modified in some cases. For example, a numerical model may use 6371,000 as the radius of the earth, while here 6371,200 is used as default. We need to add a constants module, so that these constants can be modified by users across the whole package, based on their needs.

[miniufo]

After some attempts, I decided to use default_mParams to store Rearth, Omega, and g for parameters of the Earth. One can change these through mParams.