Support default ChangeFinder hyperparameter setting

[takuti]

When a user did not specify any hyperparameters in the .ini file, DD have to automatically set default values of T1, T2, and k.

How to choose the default values for each metric? Unique values for all metrics may lead poor results.

TODOs:

• [x] Run model selection if k is not set
• [ ] Meaningful default (or optimal) values for r, T1 and T2

[takuti]

Idea: If k is set to zero, run model selection automatically.

[takuti]

Current default values:

• r = 0.02
• T1 = 10
• T2 = 5
• k = (use the result of model selection)