`.inverse()` not only returns the inverse, but modifies the original

[sliedes]

I think this probably requires at least documenting. I would have expected inverse to be non-destructive, and if there's something like .invert(), that to modify in place.

In [129]: type(a[0])
Out[129]: svgelements.svgelements.Group

In [130]: a[0].transform.inverse()
Out[130]: Matrix(3.7795296, 0, 0, 3.7795296, -24.5669424, 5.6692944)

In [131]: a[0].transform.inverse()
Out[131]: Matrix(0.264583190458, -0, -0, 0.264583190458, 6.5, -1.5)

In [132]: a[0].transform.inverse()
Out[132]: Matrix(3.7795296, 0, 0, 3.7795296, -24.5669424, 5.6692944)

In [133]: a[0].transform.inverse()
Out[133]: Matrix(0.264583190458, -0, -0, 0.264583190458, 6.5, -1.5)