[BUG] Critical bug when writing and reading again

[omaus]

Describe the bug
When writing a spreadsheet file and reading it again, it throws an error regardless of what has been done to the structure/file.

To Reproduce
Steps to reproduce the behavior:

1. Do this:

#r "nuget: FsSpreadsheet"
#r "nuget: FsSpreadsheet.ExcelIO"

open FsSpreadsheet
open FsSpreadsheet.ExcelIO

let path = <yourPath>

let wb = new FsWorkbook()

wb.InitWorksheet "empty"

FsWorkbook.toFile path wb

let wbFromFile = FsWorkbook.fromXlsxFile path

2. See error:

System.IO.IOException: Cannot modify readonly container
   at System.IO.Packaging.Package.ThrowIfReadOnly()
   at System.IO.Packaging.Package.CreatePart(Uri partUri, String contentType, CompressionOption compressionOption)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPackage.CreateMetroPart(Uri partUri, String contentType)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPart.CreateInternal(OpenXmlPackage openXmlPackage, OpenXmlPart parent, String contentType, String targetExt)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.InitPart[T](T newPart, String contentType, String id)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.InitPart[T](T newPart, String contentType)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.AddNewPartInternal[T]()
   at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.AddNewPart[T]()
   at FsSpreadsheet.ExcelIO.Spreadsheet.getOrInitSharedStringTablePart(SpreadsheetDocument spreadsheetDocument)
   at FsSpreadsheet.ExcelIO.Spreadsheet.getCellsBySheet(Sheet sheet, SpreadsheetDocument spreadsheetDocument)
   at FsSpreadsheet.ExcelIO.Spreadsheet.getCellsBySheetID(String sheetID, SpreadsheetDocument spreadsheetDocument)
   at FsSpreadsheet.ExcelIO.FsExtensions.sheets@182.Invoke(Sheet xlsxSheet)
   at Microsoft.FSharp.Collections.Internal.IEnumerator.map@128.DoMoveNext(b& curr) in D:\a\_work\1\s\src\FSharp.Core\seq.fs:line 134
   at Microsoft.FSharp.Collections.Internal.IEnumerator.MapEnumerator`1.System.Collections.IEnumerator.MoveNext() in D:\a\_work\1\s\src\FSharp.Core\seq.fs:line 117
   at Microsoft.FSharp.Collections.SeqModule.Fold[T,TState](FSharpFunc`2 folder, TState state, IEnumerable`1 source) in D:\a\_work\1\s\src\FSharp.Core\seq.fs:line 914
   at FsSpreadsheet.ExcelIO.FsExtensions.FsWorkbook.fromXlsxFile.Static(String filePath)
   at <StartupCode$FSI_0006>.$FSI_0006.main@() in C:\Users\olive\Untitled-2:line 15
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
Stopped due to error

Expected behavior
Reading should work as expected.

Additional context
It seems to be an error produced by the writer, not the reader. When reading files that were created by Excel, no error happens.

[HLWeil]

Currently working on it

[HLWeil]

fixed in #40