Extending min depth for plugins within a feature

[alanjyu]

To address the issue mentioned in #790, I extended the value of the default minimum depth of a plugin (e.g., compositional field, velocity, temperature, etc.) to the negative maximum value of double numbers. I also overhauled the depth constraint for the continental model so that the plugin is always within the depth range of the feature (the default minimum depth of a feature is still zero).

I plan to extend this feature to other features depending on their setup.

I also renamed the variables min_depth and max_depth to min_depth_entry and max_depth_entry, respectively, to reflect that they are user defined.