modify default branch to main

[AlFalahTaieb]

Changing the base branch name from master to main.

GitHub default branch is now main instead of master, by changing it to main, this will avoid creating a new branch when we write a new idea, it will directly push to the main branch instead of creating a master branch.

[simeg]

@AlFalahTaieb Thanks! This change definitely makes sense. However this will break the program for current users. Have you given any thought on how to deal with that?

A major version bump is justified but perhaps there's more we can do to help users "migrate".

[simeg]

After reading #88 I can't think of a good reason to have the branch configurable. I will remove it and default to main.

The breaking problem is still there so curious to hear your thoughts @AlFalahTaieb

[simeg]

This will be fixed in #90.