Add lazy_minus

[blegat]

This enable the user to save useless copy such as here and here.

[codecov]

Codecov Report

Merging #13 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   99.23%   99.24%   +<.01%     
==========================================
  Files           5        5              
  Lines         262      264       +2     
==========================================
+ Hits          260      262       +2     
  Misses          2        2

Impacted Files	Coverage Δ
src/abstract.jl	98.38% <ø> (ø)	⬆️
src/dematos.jl	100% <100%> (ø)	⬆️
src/avg.jl	100% <100%> (ø)	⬆️
src/decay.jl	100% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+0.006%) to 99.242% when pulling b23537a on lazy_minus into 8b5130a on master.

[coveralls]

Coverage increased (+0.006%) to 99.242% when pulling b23537a on lazy_minus into 8b5130a on master.

[coveralls]

Coverage increased (+0.006%) to 99.242% when pulling b23537a on lazy_minus into 8b5130a on master.

[coveralls]

Coverage increased (+0.006%) to 99.242% when pulling b23537a on lazy_minus into 8b5130a on master.

[frapac]

Ok to merge!