Adding downstream tracks for the tau to mugamma lines

Adding downstream electrons for the soft gamma reconstruction in the \tau\rightarrow \mu \gamma lines. Given the nature of this decay, downstream electrons are also suitable for reconstructing the soft gamma.

The expected changes were calculated running over 100K expected_2024_min_bias_hlt1_filtered_v2 events and are summarized in the following table:

line	rate reference [Hz]	expect. rate [Hz]	bw reference [KB/s]	expect. bw [KB/s]
Hlt2RD_TauToMuGamma_EE	5.7	8.92	31.2	44.6
Hlt2RD_TauToMuGamma_EE_MVA	9.6	17.84	57.9	89.2