Check that axis min/max values are in line with the spec

[mjlagattuta]

Observed behaviour

While working on Hepta Slab, my lightest instance was nan-canonical "Hairline" and sat at 0 on the weight axis in order to maintain even steps of 100. The OT spec itself says the range for this axis is 1-1000. The font builds fine even though 0 is outside the range, and there were no fvar warns in fontbakery.

When testing the font on Windows Word this was an issue since the Hairline instance failed to properly render, and was actually thicker then the Thin instance. It rendered as expected once the minimum was set to 1 in accordance with the spec.

Expected behaviour

I would expect FontBakery to note when axis min/maxes or instance values fall outside ranges noted in the spec.

Resources and exact process needed to replicate

Hepta with min of 1 @ commit 01ca19d
Hepta with min of 0 @ commit bb93812