fix: compute quartile indices dynamically instead of hardcoding 25/75

[shanboss]

Description

Fixes hardcoded Q1/Q3 indices (24/25, 74/75) in barometer calibration
outlier detection. These were hardcoded assuming a vector of exactly
100 elements, which breaks when baro_calibration_count is changed
from its default of 100.

Changes

• Compute q1_index and q3_index dynamically based on the size of
  init_static_vector_, instead of hardcoded constants.

Testing

• Verified the new formula produces identical results to the original
  hardcoded indices when n=100 (no behavior change at default config)
• Verified no out-of-bounds access / correct scaling for n=50 and n=200
  via a standalone test of the quartile logic

Closes #95

[JMoore5353]

Thanks for the contribution @shanboss!

@iandareid I pinged you on this since you are probably code owner of ROSplane... If you think it looks good, go ahead and merge it.