extra functionality

[martindurant]

• from_process source

• slice operator

• docstring clarifications

[codecov-io]

Codecov Report

Merging #241 into master will decrease coverage by 0.04%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
- Coverage   93.86%   93.82%   -0.05%     
==========================================
  Files          13       13              
  Lines        1566     1604      +38     
==========================================
+ Hits         1470     1505      +35     
- Misses         96       99       +3

Impacted Files	Coverage Δ
streamz/collection.py	94.44% <100%> (ø)	⬆️
streamz/sources.py	95.3% <92.1%> (-0.47%)	⬇️

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