Add Base.copy(::Node) and reflect this in docs and tests

[TimG1964]

I've added a simple function to recursively copy a node and all its children. I've added some tests for this, too.

The text on the Readme page currently says:

• Node is an immutable type. However, you can easily create a copy with one or more field values changed by using the Node(::Node; kw...) constructor where kw are the fields you want to change. For example:

node = XML.Element("tag", XML.Text("child"))

simple_value(node)
# "child"

node2 = Node(node, children=XML.Text("changed"))

simple_value(node2)
# "changed"

This seems to be a bit out of date. The Node() function seems to treat arguments as new children and keywords as new/changed attributes. The example presented now seems to create an attribute called "children" rather than changing the child node. I've suggested some changes to the Readme to reflect this.

(Hope this goes OK this time!)

[TimG1964]

Reflecting on this discussion on Discourse, maybe this new function shouldn't overload Base.copy. Perhaps XML.copynode would be a better name.

[joshday]

Hmm, I don't think I like having an XML.copynode, creating three different ways (including copy and deepcopy) to create a copy.

With all the concerns I've seen written about deepcopy, I've never experienced any unwanted effects from it.

[TimG1964]

OK, I understand. Just to note, though, that deepcopy is more expensive than this proposed function:

julia> f=XLSX.openxlsx(raw"C:\Users\tim\OneDrive\Documents\Julia\XLSX\XLSX.jl\data\customXml.xlsx")
XLSXFile("C:\Users\tim\OneDrive\Documents\Julia\XLSX\XLSX.jl\data\customXml.xlsx") containing 2 Worksheets
            sheetname size          range        
-------------------------------------------------
              Mock-up 116x11        A1:K116      
     Document History 17x3          A1:C17

julia> @time begin; for i=1:10; deepcopy(f.data["xl/workbook.xml"]); end; end
  0.000710 seconds (14.20 k allocations: 765.736 KiB)

julia> @time begin; for i=1:10; XLSX.copynode(f.data["xl/workbook.xml"]); end; end
  0.000143 seconds (3.77 k allocations: 193.594 KiB)

Difference may only be marginal for most use cases, though.

[TimG1964]

I captured the issue of out of date text on the Readme page in #44.