Allows overwriting default line columns

[inoda]

This change is 

[CLAassistant]

All committers have signed the CLA.

[inoda]

This addresses an issue I opened: #187

Example usage:

	cell1 := pdfInvoice.NewColumn("Description")
	cell2 := pdfInvoice.NewColumn("Amount")
	cell2.Alignment = creator.CellHorizontalAlignmentRight
	pdf.SetColumns([]*creator.InvoiceCell{cell1, cell2})

[adrg]

Looks good to me. This method should ideally be used before adding any lines to the table. Otherwise, it could produce unwanted results (if the number of columns does not match the number of cells in a line). However, it's the responsibility of the user, so I think it's ok. It's very easy to construct a slice of columns by using the NewColumn method of the invoice.

[inoda]

@adrg @gunnsth I updated the code comment to reflect that potential issue! How long before this is merged and usable? (we are blocked by this)

[codecov]

Codecov Report

Merging #188 into development will decrease coverage by 5.49%.
The diff coverage is 0%.

@@              Coverage Diff               @@
##           development     #188     +/-   ##
==============================================
- Coverage        57.57%   52.07%   -5.5%     
==============================================
  Files              189      189             
  Lines            34460    34462      +2     
==============================================
- Hits             19839    17945   -1894     
- Misses           12073    14177   +2104     
+ Partials          2548     2340    -208

Impacted Files	Coverage Δ
creator/invoice.go	88.43% <0%> (-0.43%)	⬇️
internal/e2etest/memory_measure.go	0% <0%> (-100%)	⬇️
internal/e2etest/validate.go	0% <0%> (-70%)	⬇️
model/flatten.go	0% <0%> (-64.23%)	⬇️
internal/e2etest/utils.go	0% <0%> (-60%)	⬇️
annotator/field_appearance.go	10.87% <0%> (-39.75%)	⬇️
contentstream/processor.go	17.07% <0%> (-34.46%)	⬇️
extractor/text.go	40.54% <0%> (-24.95%)	⬇️
model/colorspace.go	10.92% <0%> (-17.07%)	⬇️
model/functions.go	18.76% <0%> (-16.78%)	⬇️
... and 45 more

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