SSM_v.0.1: Add a new function to read MODFLOW head and define the bottom of the soil layer that include that head

[MostafaGomaa93]

Description

The changes are as follows:
1- a new function "TIME_INTERPOLATION" is added. It reads MODFLOW heads and defines the bottom of the soil layer that includes that head. Currently, the output of the function is not used yet but will be used later in the next modification steps.
2- Corresponding modifications are in getModelSettings.m and define_constants.m files

I have included in the code the description of the variables I know so far, but not sure about all of them yet. The function must be checked by @Lianyu-Yu (the original author), and further checks by other members. Still a room of changes is expected after I got responses of my questions in issue 217 from @Lianyu-Yu

Checklist

• [✓] Add a reference to related issues -> issue 217
• [✓] @mentions of the person or team responsible for reviewing proposed changes -> @SarahAlidoost, @Lianyu-Yu, @BSchilperoort, @yijianzeng
• [✓] This pull request has a descriptive title.
• [✓] Code is written according to the guidelines.
• The checks by MISS_HIT style checker and linter, below the pull request, are successful (green).
• Documentation is available.
• Add changes to the changelog file under section Unreleased.
• [✓ ] Model runs successfully.
• Ask a meinatainer to re-generate exe file if matlab codes are changed. About how to create an exe file, see exe readme.