GradientFill stops should not have position determined by `enumerate`
Created originally on Bitbucket by jnothman
The following gradientFill
renders fine in Excel
<gradientFill type="linear">
<stop position="0"><color rgb="FFC6C6C6" /></stop>
<stop position=".3"><color rgb="FFFFFFFF" /></stop>
<stop position="1"><color rgb="FFFFC6C6" /></stop>
</gradientFill>
OpenPyxl currently only supports a list of gradient stops being colours, which are then enumerated to determine their position. This implies that at most two stops (the endpoints) are acceptable.
It should be possible to specify stops as (position, color)
.