Add multiplier and allow_missing macro use

[lrennels]

This PR (1) adds a multiplier build in component to help with FUND-FAIR unit conversion and (2) per suggestion from #560 adds an @allow_missing wrapper to the adder and multiplier components, which also simplifies MimiFUND-MimiFAIR-Flat.

This means that if the multiplier component:

@defcomp multiplier begin

    multiply  = Parameter(index=[time])
    input       = Parameter(index=[time])
    output      = Variable(index=[time])

    function run_timestep(p, v, d, t)
        v.output[t] = @allow_missing(p.input[t]) * p.multiply[t]
    end
end

accesses a missing in p.input[t] it will not throw an error but just pass the value through. Sometimes needing this can be avoid with setting a first and last for the multiplier component, but sometimes there's still an irritating off-by-one issue. For example in MimiFUND-MimiFAIR-Fla we have

set_param!(m, :multiplier, :multiplier, 1/1000) # convert Mtons CO₂ coming out of FUND to Gtons CO₂ going into FAIR
connect_param!(m, :multiplier, :input, :emissions, :mco2)
connect_param!(m, :co2_cycle, :E_CO₂, :multiplier, :output, FAIR_CO₂_backup, backup_offset = 1)

where the :mco2 from FUND's :emission is missing for the first year FUND runs (1950), and thus we would need multiplier to run from FUND_first + 1 to FUND_last ... or do something fancy with backups which also works. In my opinion we want these to be sort of no-brainer pass-through components that don't do anything fancy so I'd rather just do @allow_missing but we can discuss if you'd like.

[codecov]

Codecov Report

Merging #816 (36f7eed) into master (cf87a03) will decrease coverage by 0.13%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
- Coverage   84.25%   84.12%   -0.14%     
==========================================
  Files          39       40       +1     
  Lines        3551     3552       +1     
==========================================
- Hits         2992     2988       -4     
- Misses        559      564       +5     

Flag	Coverage Δ
unittests	84.12% <0.00%> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Mimi.jl	100.00% <ø> (ø)
src/components/adder.jl	0.00% <0.00%> (ø)
src/components/multiplier.jl	0.00% <0.00%> (ø)
src/core/defcomp.jl	82.48% <ø> (+0.46%)	⬆️
src/explorer/buildspecs.jl	92.00% <0.00%> (-2.67%)	⬇️

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[arnavgautam]

Just minor comments on the tutorial language

[arnavgautam]

The second parameter here is 'multiply' right?

[arnavgautam]

Are there three "built-in" components now?