Add rsqrt function and test cases

[yashrajgupta]

Adding rsqrt function from Recommended Elementary function.

[coveralls]

Coverage increased (+0.07%) to 82.506% when pulling b6fcc80 on yashrajgupta:rsqrt_function_patch into 9a38d80 on JuliaIntervals:master.

[coveralls]

Coverage increased (+0.03%) to 90.51% when pulling 5b302fb on yashrajgupta:rsqrt_function_patch into 1516c5e on JuliaIntervals:master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1516c5e). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #268   +/-   ##
========================================
  Coverage          ?   82.5%           
========================================
  Files             ?      25           
  Lines             ?    1229           
  Branches          ?       0           
========================================
  Hits              ?    1014           
  Misses            ?     215           
  Partials          ?       0

Impacted Files	Coverage Δ
src/IntervalArithmetic.jl	50% <ø> (ø)
src/intervals/functions.jl	82.72% <100%> (ø)

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[dpsanders]

I'm not sure that rsqrt is a good name for this function. Maybe invsqrt? In fact, is this function really necessary? I guess the idea is that it should be more accurate than inv(sqrt(X)).

[yashrajgupta]

Actually the function is there with the same name in the document as well in octave. The implementation should give the tighest interval.

[lbenet]

In the standard it appears as rSqrt; my guess is that "r" stands for "reciprocal". Note that the standard does not impose a specific name.

I agree with David that rsqrt is maybe not a good name. What about invsqrt, recipsqrt, reciprocalsqrt, or perhaps add some _ to make it more readable (e.g., reciprocal_sqrt).

Independently of the name, one more suggestion: add docstrings and also an entry into the documentation.

[yashrajgupta]

Ok, I'll change the name and do the suggested.

[lbenet]

Any progress on this?

[yashrajgupta]

Yeah I have changed the name and added a doc string. You can review it.

[dpsanders]

Could you please rebase?