Add lower bound

[mschilli87]

new attempt of #85

This has the + 1 instead of the - 1 which I still believe to be correct (see #90).

But it works for the current test case and the 2nd step function is definitely needed, so I push it as it is for now.

I suggest to keep playing around a bit before to see if this finally solves #66 before merging this to the master branch.

[codecov-io]

Current coverage is 75.27%

Merging #91 into add_lower_bound will not change coverage

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