add support for different frequencies on DS3231 INT/SQW pin

[lagg070988]

Description

add support for different frequencies on DS3231 INT/SQW pin

Fixes #18 (and #9)

Type of change

• [ *] New feature (non-breaking change which adds functionality)
• [ *] This change requires a documentation update

How Has This Been Tested?

Test Configuration:

• Arduino version:
• Arduino Hardware: UNO, NANO
• SDK: Arduino IDE
• Development sytem: Ubuntu 20.04

Checklist:

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

[SV-Zanshin]

Due to other changes, I am going to close this pull request but add the changes to the main branch myself.