Incremental performance

[TimG1964]

Builds on everything in #298 and adds the following:

• Minor refactor of setFormatAttribute family of functions to more than double speed.
• Reduce reliance on eachrow to iterate through sheets. Indexing into the worksheet cache (where possible) is generally faster.
• Make clearer separation between iterating a file stream vs through the cache.
• Refactor setConditionalFormat to pass a Dict of keyword options as discussed here to speed up compilation a little.
• Add a copysheet! function to make a copy of a worksheet in an XLSXFile.
• Change the XLSXFile.source of XLSXFiles created with newxlsx to be located in pwd() rather than _relocatable_data_path
• Include code proposed in issue 299, empty v element #300 and Unescape formula, issue 301 #302

[codecov]

Codecov Report

Attention: Patch coverage is 90.75819% with 206 lines in your changes missing coverage. Please review.

Project coverage is 93.54%. Comparing base (f4767c4) to head (49aac9a).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
src/cellformat-helpers.jl	93.57%	42 Missing ⚠️
src/worksheet.jl	87.80%	30 Missing ⚠️
src/cellref.jl	87.00%	26 Missing ⚠️
src/workbook.jl	86.09%	21 Missing ⚠️
src/write.jl	94.36%	21 Missing ⚠️
src/read.jl	88.97%	14 Missing ⚠️
src/cell.jl	54.54%	10 Missing ⚠️
src/styles.jl	81.13%	10 Missing ⚠️
src/conditional-format-helpers.jl	95.96%	9 Missing ⚠️
src/relationship.jl	77.27%	5 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #304      +/-   ##
==========================================
- Coverage   95.02%   93.54%   -1.48%     
==========================================
  Files          15       19       +4     
  Lines        2009     5715    +3706     
==========================================
+ Hits         1909     5346    +3437     
- Misses        100      369     +269     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[TimG1964]

I've tried to work around issue 43 in XML.jl but can't do this completely. There was an issue in the handling of blank (whitespace only) strings in XLSX, too (#308), which I've tried to fix.

• Can now write cells containing only whitespace characters or with leading or trailing whitespace.
• Cannot see these things in cells read in from existing Excel files - XML removes leading whitespace even if xml:space="preserve" is specified.
• Thus a cell containing " " will be read in as missing. A cell containing " hello" will become "hello".

[TimG1964]

I've made a PR to XML.jl to try to fix the issue with xml:space-"preserve". I'll add some tests for this if the PR gets merged.