Add binder

[malmans2]

Closes #12

I've added the badge with the link to the binder environment. https://mybinder.org/v2/gh/MillionConcepts/lhorizon/main?filepath=lhorizon%2Fexamples

However, the link above will only work after merging this PR (please check after merging). Until then, you can test binder using this link: https://mybinder.org/v2/gh/malmans2/lhorizon/add_binder?filepath=lhorizon%2Fexamples

A couple of suggestions:

1. I think you could move the lhorizon/examples to the root directory, it doesn't need to be under /lhorizon. If you do that, you'd have to update the binder link. The link generator is here: https://mybinder.org/
2. Right now only users familiar with binder would click on the binder badge. I'd mention that the notebooks can be executed interactively in a live environment and the link in the usage section

https://github.com/openjournals/joss-reviews/issues/3495

[codecov-commenter]

Codecov Report

Merging #16 (67a73f1) into main (3c66902) will decrease coverage by 0.16%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   95.95%   95.79%   -0.17%     
==========================================
  Files          14       14              
  Lines         594      594              
==========================================
- Hits          570      569       -1     
- Misses         24       25       +1     

Impacted Files	Coverage Δ
lhorizon/lhorizon_utils.py	94.77% <0.00%> (-0.75%)	:arrow_down:

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[m-stclair]

looks cool, and I think you are right that it is better to pull the examples directory into the repo root and explicitly inform users about Binder.