Added Rabin–Karp algorithm

[pratikgl]

Added Rabin–Karp algorithm to find API

References to other Issues or PRs or Relevant literature

Fixes #402

Brief description of what is fixed or changed

In the find API implemented in the string algorithm. I have added Rabin–Karp algorithm which finds the occurrence of a query string in a text using string hashing.

Other comments

Reference: https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm

[codecov[bot]]

Codecov Report

Merging #413 (823fc7b) into master (5cf53a2) will increase coverage by 0.013%. The diff coverage is 100.000%.

:exclamation: Current head 823fc7b differs from pull request most recent head 98209e4. Consider uploading reports for the commit 98209e4 to get more accurate results

@@              Coverage Diff              @@
##            master      #413       +/-   ##
=============================================
+ Coverage   98.622%   98.635%   +0.013%     
=============================================
Files           26        26               
Lines         3411      3444       +33     
=============================================
+ Hits          3364      3397       +33     
Misses          47        47               

Impacted Files	Coverage Δ
pydatastructs/strings/algorithms.py	100.000% <100.000%> (ø)

[pratikgl]

Changes added:

1. Added _p_pow function to decrease redundancy in the code
2. Defined global variables PRIME_NUMBER, MOD

[czgdp1807]

Thanks @pratikgl