MHHW not computed when using GTSM daily maxima as ESL data

When setting```esl_data: "gtsm_dmax"``` in ```config.yml```, MHHW is not computed from the data, and ```below_gpd_threshold: "mhhw"``` will be ignored. Computation of MHHW and storage in ```esl_statistics``` should therefore be added to the relevant function in ```esl_analysis.py```.