Allow recipes to set previous operation

[gustaphe]

Previously, recipes had no way of affecting the previous operation. With this (somewhat ugly) change, a type that fundamentally represents for instance a multiplication or sum can tell latexify, so that things like parentheses work out right.

Part of fixing #280

TODO

• [x] Documentation
• [ ] Maybe more tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.81%. Comparing base (6af2a54) to head (1577e93). Report is 2 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #292 +/- ## ========================================== - Coverage 81.78% 77.81% -3.97% ========================================== Files 21 21 Lines 818 834 +16 ========================================== - Hits 669 649 -20 - Misses 149 185 +36 ```

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[gustaphe]

Without this,

julia> struct SimpleLength
x
end

julia> @latexrecipe function f(x::SimpleLength)
return Expr(:latexifymerge, x.x, "\\;\\mathrm{m}")
end

julia> @latexify $(SimpleLength(3))^2

3\;\mathrm{m}^2

With this

julia> @latexrecipe function f(x::SimpleLength)
operation := :*
return Expr(:latexifymerge, x.x, "\\;\\mathrm{m}")
end

julia> @latexify $(SimpleLength(3))^2

\left( 3\;\mathrm{m} \right)^{2}