Added equals() method to DateTime

[davidlehrian]

Description

Added a method to check if two DateTimes hold equal values.

Fixes # (issue) enhancement

Type of change

• [X] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added test to RegressionTests.ino (and fixed an issue with the battery backup test while I was in there)

Test Configuration:

• Arduino version: 1.8.13
• Arduino Hardware: Arduino Nano
• SDK: Arduino IDE
• Development system: Windows

Checklist:

• [ ] The changes made link back to an existing issue number
• [X] I have performed a self-review of my own code
• [X] My code follows the style guidelines of this project
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] The code adheres to the Google Style Guide
• [X] The code follows the existing program documentation style using doxygen
• [ ] I have made corresponding changes to the documentation / Wiki Page(s)
• [X] My changes generate no new warnings
• [ ] The automated TRAVIS-CI run has a status of "passed"
• [X] I have checked potential areas where regression errors could occur and have found no issues
• [X] 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