Add new c'tors to LinExpr

[hedtke]

Add new c'tors to LinExpr to speed-up the creation of expressions with multiple variables.

Fixes #32

This is an alternative implementation of #34

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.13%. Comparing base (d9061e8) to head (f9678e5).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   95.62%   96.13%   +0.50%     
==========================================
  Files          10       10              
  Lines         183      207      +24     
  Branches       48       60      +12     
==========================================
+ Hits          175      199      +24     
  Misses          3        3              
  Partials        5        5              

Files with missing lines	Coverage Δ
include/scippp/lin_expr.hpp	100.00% <100.00%> (ø)
source/lin_expr.cpp	100.00% <100.00%> (ø)

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[hedtke]

@TonyCongqianWang you might want to have a look into this

[TonyCongqianWang]

I'll try to do it tomorrow!

[TonyCongqianWang]

Looks good to me. Not having spans should not be a big drawback since copying variables is not going to be very expensive. Using std::vector should allow enough flexibility for most use cases

[hedtke]

@erogelio18 ready for next review round

[hedtke]

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