increase length of vector for Sum checkbox icon

[pixelzoom]

While looking at the sim via Zoom with @ariel-phet, he suggested increasing length of vector for Sum checkbox icon. This was prompt by the longer vector icons that we used for the 'show base vectors' checkbox in #153.

[pixelzoom]

Increased vector length from 22 to 30. @arouinfar please review in master. Before after screenshots below.

Before:

After:

[arouinfar]

Looks great @pixelzoom!