Recalibrate GRAD as 0..1

[davelab6]

The Google Fonts team wants to seek opportunities to simplify in order to drive adoption of variable fonts, and as a library we want to offer the same axes across families where possible.

Currently we are using the GRAD axis proposed by TypeNetwork, which explains why -1..1 is preferred as the global max range:

https://github.com/TypeNetwork/Opentype-1.8-Axis-Proposal/blob/gh-pages/proposal/Grade/ProposalSummary.md

Here the grade is only positive, so I request recalibrating the axis from 1..500 to 0..1

[psb1558]

This one is easy.

[psb1558]

This is done, and it seems to work well.