Vibronic Effects in the 1<sup>1</sup>B<sub>u</sub>(1<sup>1</sup>B<sub>2</sub>) Excited Singlet States of Oligothiophenes. Fluorescence Study of the 1<sup>1</sup>A<sub>g</sub>(1<sup>1</sup>A<sub>1</sub>) ← 1<sup>1</sup>B<sub>u</sub>(1<sup>1</sup>B<sub>2</sub>) Transition in Terms of DFT, TDDFT, and CASSCF Methods