chore: change branch names

[edkerk]

Main improvements in this PR:

• refactor:
  • rename master and devel branches to main and develop (closes #258)

I hereby confirm that I have:

• [x] Tested my code with all requirements for running the model
• [x] Selected develop as a target branch (top left drop-down menu)
• [x] If needed, asked first in the Gitter chat room about this PR

[edkerk]

Branches are now renamed.