chainladder 0.4.0

Enhancements:

• Added better datetime management when instantiating a triangle
• Added json serializers for Triangle as well as the estimators and Pipeline. These can be accessed with the to_json method and cl.read_json
• Added DevelopmentConstant estimator that takes LDF or CDF patterns as a dictionary instead of calculating from a triangle. This is useful for incorporating industry patterns into an analysis
• General improved functionality for malformed triangles
• improved memory management by eliminating unnecessary usage of deepcopy

Bug Fixes:

• Fixed an issue where LDFs were calculated incorrectly for asymmetric triangles
• Fixed a bug where slicers weren't working properly after particular mutations (e.g. adding a new column)
• Arithmetic on triangles with different origin ranges will now take the union of the origin periods. Previously the intersection was taken.