Enforce overlap of oligo-d(T) & poly(A) tail

[mschilli87]

The model currently allows the oligo-d(T) to overlap by one or more nucleotides with the poly(A) tail.

So far, the simulations allowed a zero-overlap too.

The changes introduced by this pull request not only fix this discrepancy, but also enable simulations with larger minimal overlaps than the current default of 1. Note however, that this is not yet implemented on the model side (see #87).

[codecov-io]

Current coverage is 75.27%

Merging #88 into simulation will decrease coverage by 0.33%

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- Hits              276        271     -5   
  Misses             89         89          
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[mschilli87]

I merged the current master branch into the simulation branch to continue working on #66 eventually. So we can leave the branch around after merging this to master.

[mschilli87]

Note that the only new commit in this pull request is 46062bb.