[WIP] 3 arg update_func

[vpuri3]

fix #202

Operators should accept a single update_func with two methods A = update_func(u, p, t), and update_func(A, u, p, t) # = nothing. The DEFAULT_UPDATE_FUNC shall return nothing to indicate that no update has taken place.

TO DO

• remove update_func!
• add 3 arg DEFAULT_UPDATE_FUNC
• bump version to 0.4

[codecov]

Codecov Report

Merging #206 (b63bfb2) into master (3e808d1) will decrease coverage by 1.24%.
Report is 3 commits behind head on master.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   70.08%   68.84%   -1.24%     
==========================================
  Files          10       10              
  Lines        1588     1589       +1     
==========================================
- Hits         1113     1094      -19     
- Misses        475      495      +20     

Files	Coverage Δ
src/matrix.jl	63.91% <ø> (-4.13%)	⬇️
src/interface.jl	40.36% <0.00%> (-6.86%)	⬇️

... and 2 files with indirect coverage changes

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[gaurav-arya]

Hey, what's the status of this? It could be good to merge soon-ish to minimize the amount of code in the old style we'll have to adjust