Updating yaml reader code to match new clix-meta specification. (fixes #261)

• Copied new clix-meta v0.4.0 yaml files (index_definitions.yml and variables.yml) into climix/etc/ folder
• Deleted climix/etc/b4est_indices.yml
• Removed IndexDefinition.period member and added IndexDefinition.default_period with type str
• Changed units in CountLevelCrossings and CountOccurences index function classes from "days" to "1"
• Fixed index name placeholder/parameter handling (interpret parameter as integer) after change in clix meta yaml format.