Update to clix-meta v0.5.1

Closes #298 (closed)

• Updated index_definitions.yml and variables.yml to the clix-meta v0.5.1 versions.
• Updated metadata.yml accordingly.
• Changed argument lists for affected index functions: diurnal_temperature_range, running_statistics, spell_length, statistics, thresholded_running_statistic and thresholded_statistic. I've tested one index of each index function except for running_statistics which is currently not used by any index in index_definitions.yml.
• Added a ParameterTimeRange dataclass to read parameter type time_range used for reference_period by some indices. Also updated the corresponding index functions to parse the reference_period in iso format. Note: this affects !208 (merged), which will require a rebase and some modification.