BUG/STY: update code to be compliant with numpy, pandas, and pysat development paths

[jklenzing]

Description

Numpy 1.18.1 throws a TypeError when trying to interpret floats as integers. Updates the test routines to no longer pass through floats to numpy functions when integers are expected.

Pandas 2.0.0 will no longer carry around datetime, but asks users to import directly from datetime instead. Most of this was fixed in a previous pull, but the demo code was not updated at the time. This brings it in line with code updates.

Also updates demos to use custom.attach rather than custom.add in line with pysat 3.0.0 changes.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Running demo codes locally. Also, pytest on full package.

Checklist:

• [x] Make sure you are merging into the develop (not master) branch
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] Add a note to CHANGELOG.md, summarizing the changes

[jklenzing]

Updates to code: added the conda manager to Travis CI, removed the python 3.8 tests as cannot get pandas to work there.

[jklenzing]

Part of the conda update went straight to develop, but in a version that doesn't work. This fixes those bugs too.