Features/284 add to exponents and logarithms

[krajsek]

Add log1p + unit tests + remove some duplicate unit tests

[codecov-io]

Codecov Report

Merging #327 into master will decrease coverage by 0.01%.
The diff coverage is 98%.

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
- Coverage   96.78%   96.77%   -0.02%     
==========================================
  Files          53       53              
  Lines        8128     8216      +88     
==========================================
+ Hits         7867     7951      +84     
- Misses        261      265       +4

Impacted Files	Coverage Δ
heat/core/exponential.py	100% <100%> (ø)	⬆️
heat/core/tests/test_rounding.py	100% <100%> (ø)	⬆️
heat/core/tests/test_exponential.py	100% <100%> (ø)	⬆️
heat/core/dndarray.py	95.03% <100%> (+0.05%)	⬆️
heat/core/rounding.py	93.93% <100%> (+0.39%)	⬆️
heat/core/communication.py	92.64% <75%> (-1.43%)	⬇️

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[coquelin77]

looks good. Thanks!