Refactor some code

[lokeshh]

I had a look at some places where code can be improved while documenting the functions. This PR is for that purpose.

[lokeshh]

@v0dro I see your comment to remove const_get. Is there a way to do it without using const_get?

[v0dro]

http://stackoverflow.com/a/24420705/3992369

[v0dro]

Remove the need of chaining basically.

[lokeshh]

Is the current change fine or should I change to something like Object.const_get('Daru::Vector')?

[v0dro]

Keep a single const_get. In other words go with the SO answer.

[lokeshh]

Changed degree_of_freedom to degrees_of_freedom and standard_error to standard_errors. Also added aliases so the old names could be used. I find the tests for these functions are missing.

Should I add the tests? Also should I print a error message to tell that the old names will be removed?

[v0dro]

Aren't these the tests you're looking for? Yes print an error message into $stderr.

[agisga]

I saw too that not every method is tested for every model (logistic, probit, normal etc.), and in general there is a lot of room for improvement of the tests. For example #standard_errors is only tested for logistic regression, and I think you are right that #degrees_of_freedom is never tested.
Though maybe addition of missing tests should be a separate commit (to make the commit history more organized)?

[v0dro]

Ah yes there's no tests but those for logit. And new tests should be a separate commit.

[lokeshh]

I think this is ready to be merged now. Here are the changes:

• Replace multiple const_getwith single const_get.
• Deprecate degree_of_freedom and introduce degrees_of_freedom
• Deprecate standard_error and introduce standard_errors