Add scale invariance and bounds checking

[Rupt]

Teach mt2_bisect_impl to divide out the scale to give bigger dynamic range.

Add a check for negative bounds, which can be caused by nasty negative mass inputs.

Add tests for both of these changes.

Make the template float-compatible by changing a couple of literals.

Add some formatting and comments improvments.

Affects issues #42 #43

[tpgillam]

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