Adds support to missing data for where clause

[athityakumar]

Fixes issue #246. Sample output for the building pass is shown below.

2.3.1 :002 > v= Daru::Vector.new([1,2,3,Float::NAN, nil])
 => #<Daru::Vector(5)>
   0   1
   1   2
   2   3
   3 NaN
   4 nil 
2.3.1 :003 > v.where(v.lt(4))

 => #<Daru::Vector(3)>
   0   1
   1   2
   2   3 
2.3.1 :004 > v.where(v.lt(2))

 => #<Daru::Vector(1)>
   0   1 
2.3.1 :005 > v.where(v.lt(3))

 => #<Daru::Vector(2)>
   0   1
   1   2 
2.3.1 :006 > v.where(v.gt(1))

 => #<Daru::Vector(2)>
   1   2
   2   3 

Ping @v0dro @zverok @lokeshh - Please review this PR when you're free. :smile:

[athityakumar]

Tests, as in using Travis CI or test-unit gem?

[zverok]

@athityakumar I've meant specs -- there are ton of them & you can run them locally by just rspec or rspec path/to/your/spec.rb

[v0dro]

@athityakumar I suggest you first go through the CONTRIBUTING file and understand our processes properly before submitting PRs. We're all volunteers and it saves our time if you abide by some formal processes.

[v0dro]

Also, submitting a PR without tests is not acceptable. Especially if you want to apply for GSOC.

[athityakumar]

@v0dro : Thanks for mentioning about the CONTRIBUTING.md file - I just read through it. I'll add the tests in a day, after including the other missing data types and let you know for reviewing the PR again.

[athityakumar]

@v0dro @zverok : The PR is ready for another review.

Updates -

(1) Thanks for the rspec and rubocop references. They really made testing the changes simpler. (2) Support added for other missing data types like Float:NAN and Float:NAN in string. (3) Tests have been written for Daru::Vector#where and Daru::DataFrame#where.

Any changes required?

[athityakumar]

@zverok : I've added the specific test that would have failed on the current master branch version of daru. As suggested, I've also added my test variables in letformat - however, it does seem a bit different / inconsistent with the rest of the tests in the spec/core/query_spec.rb file, as they don't use let format.

[zverok]

however, it does seem a bit different / inconsistent with the rest of the tests in the spec/core/query_spec.rb file, as they don't use let format.

Yes, part of the specs are just old and written without much attention. We want to have them rewritten in modern style with time, yet it was not done yet.

[athityakumar]

@v0dro @zverok : Made the required changes in tests. :smile: