Add functions std and var

[densmirn]

Combined 2 functions to 1 PR because one depends on the other.

[codecov]

Codecov Report

Merging #105 into master will decrease coverage by 0.12%.
The diff coverage is 47.05%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   51.45%   51.33%   -0.13%     
==========================================
  Files          19       19              
  Lines        1201     1235      +34     
  Branches      312      326      +14     
==========================================
+ Hits          618      634      +16     
- Misses        333      341       +8     
- Partials      250      260      +10     

Impacted Files	Coverage Δ
dpnp/dpnp_iface_statistics.py	43.44% <47.05%> (+1.10%)	⬆️

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