Rename author 'M. Stendel' to 'Martin Stendel'

This author contributed to AR3 and AR4:
http://www.baltex-research.eu/supplementary/BSSG27/stendel_CV_20081121.pdf

We only have a record for 'M. Stendel' in AR3 WGI Ch2 so far
(not in AR4).

The name 'Martin Stendel' appears in AR5.