compute_rset_map: fix paper reference, add a percentile method

[chraibi]

Two points on compute_rset_map (pedpy/methods/profile_calculator.py, PedPy 1.5.1):

1. Docstring reference: it says the implementation "follows Section 5.5.1" of Schröder et al. 2020 (Fire Safety Journal, doi:10.1016/j.firesaf.2020.103154). The paper has no Section 5.5.1; the RSET map is defined in §2.3, Eqs. 4–5 (or in the 2017 thesis, if that was meant).
2. Percentile method: RsetMethod offers MAX, MIN and MEAN. For pooling several runs, or for a robust per-cell RSET, a PERCENTILE option (e.g. RsetMethod.PERCENTILE with q=95) would avoid a single outlier agent setting the map.

Also worth stating in the docstring: cell edges start at walkable_area.bounds min, the output is flipped so row 0 is max y, and time is frame / frame_rate.