Conditional plotting of read length distributions vs. a summary table

[standage]

The mhpl8r pipe report includes a module for plotting read length distributions.

However, when all samples have uniform read lengths, you end up with something like...this.

This PR replaces these plots with a table of read lengths, when and only when all samples have uniform lengths.

Closes #137.

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• Changes are clearly described above
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• Any new features are tested (see the development manual for details)
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