Simplified way to set cell formulas

[adknudson]

I'd like to propose a simplified way to setting a cell formula. I see that in #120 the proposed way to set a cell formula is to do

sheet["A1"] = ""
(XLSX.getcell(sheet, "A1")).formula = "=ROUND(3.14159, 2)"

I see that sheet["A1"] = "" is necessary because sheet["A1"] can return an EmptyCell which has no fields.

I also see that there is the AbstractFormula type (which isn't yet being used in the Cell type in v0.9.0). Is this going to be used in future versions? If so, I can imagine an interface that looks like this:

abstract type AbstractFormula end

formula(f::AbstractFormula) = f.formula

struct FormulaA1 <: AbstractFormula 
    formula::String
end

Base.convert(::Type{FormulaA1}, v::FormulaA1) = v

function Base.convert(::Type{FormulaR1C1}, v::FormulaA1)
    # convert A1 to R1C1
end

struct FormulaR1C1 <: AbstractFormula 
    formula::String
end

function Base.convert(::Type{FormulaA1}, v::FormulaR1C1)
    # convert R1C1 to A1
end

Base.convert(::Type{FormulaR1C1}, v::FormulaR1C1) = v


function setformula!(ws::XLSX.Worksheet, ref::XLSX.CellRef, v::AbstractFormula)
    f = formula(convert(::FormulaA1, v))

    cell = XLSX.getcell(ws, ref)
    
    if cell isa XLSX.EmptyCell
        cell = XLSX.Cell(ref, "", "", "", f)
        XLSX.setdata!(ws, cell)
    else
        cell.formula = f
    end
    
    return cell
end

I'm not yet too familiar with internals of this package, but with some direction I'd be willing to prototype this out and open a PR. Let me know what you think, and thanks for this package!