Precise Stark-effect measurements of the (4f<sup>6</sup>6s6p+4f <sup>5</sup>5d6s<sup>2</sup>)<sup>7</sup>F<sub>1</sub> <sup>0</sup>-4f <sup>6</sup>6s<sup>2</sup> <sup>7</sup>F<sub>1</sub> transition in Sm I (5720.19 AA)