Define Daru::Index#[] to take only index value as argument

[ananyo2012]

Daru::Index#[] now only takes index values and returns positions only. If index is not found it returns nil

2.3.0 :001 > index = Daru::Index.new [:one, :two, :three, :four] => #<Daru::Index(4): {one, two, three, four}> 
2.3.0 :002 > index[:one..:three] 
 => [0, 1, 2] 
2.3.0 :003 > index[:five..:six]
 => nil 
2.3.0 :004 > index[:one, 0]
 => [0, nil] 
2.3.0 :005 > index[:two]
 => 1 
2.3.0 :006 > index[2]
 => nil 

[ananyo2012]

@v0dro @zverok Review please.

[ananyo2012]

Hi @zverok @v0dro Just thought to ping ypu since its been 4 days since the PR.

[zverok]

I'll review it tomorrow. Lots of stuff is going on currently, 4 days of PR review delay is not that much by OSS standards :trollface:

[athityakumar]

@zverok - The troll though :joy:

[ananyo2012]

Hi @zverok @v0dro . I had my final exams so couldn't check on this. I see there are some merge conflicts. Will solve it and put it for review again

[ananyo2012]

Please have a look at it once again.

[zverok]

Thanks for your hard work!

[ananyo2012]

Thanks! Will take up a new issue soon.