Make it so we can find RAVEN without pandas.

Pull Request Description

Closes #291

Lazy loads pandas so it is not needed for entire _util.py file.

The following review must be completed by an authorized member of the Change Control Board.

[x] 1. Review all computer code.
[x] 2. If any changes occur to the input syntax, there must be an accompanying change to the user manual and xsd schema. If the input syntax change deprecates existing input files, a conversion script needs to be added (see Conversion Scripts).
[x] 3. Make sure the Python code and commenting standards are respected (camelBack, etc.) - See on the wiki for details.
[x] 4. Automated Tests should pass.
[x] 5. If significant functionality is added, there must be tests added to check this. Tests should cover all possible options. Multiple short tests are preferred over one large tes.
[x] 6. If the change modifies or adds a requirement or a requirement based test case, the Change Control Board's Chair or designee also needs to approve the change. The requirements and the requirements test shall be in sync.
[x] 7. The merge request must reference an issue. If the issue is closed, the issue close checklist shall be done.
[x] 8. If an analytic test is changed/added, the the analytic documentation must be updated/added.
[x] 9. If any test used as a basis for documentation examples have been changed, the associated documentation must be reviewed and assured the text matches the example.