Extend COOAssembler to also handle vector assembly

[fredrikekre]

Follow up to #916.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 93.71%. Comparing base (c6e65e0) to head (a6eb9ce).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/assembler.jl	87.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1058      +/-   ##
==========================================
- Coverage   93.73%   93.71%   -0.03%     
==========================================
  Files          39       39              
  Lines        6017     6042      +25     
==========================================
+ Hits         5640     5662      +22     
- Misses        377      380       +3     

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[fredrikekre]

In order to make progress with 1.0 I did this:

users who wants the COOAssembler should simply call assembler = COOAssembler(args...; kwargs...)

for now and we can decide later if we are introducing a COOMatrix or something to call start_assemble on (I started on something like that in #566 but don't remember why I didn't like it).