Description
func (f *File) GetComments() (comments map[string][]Comment) returns wrong mapping between worksheets and comments.

This problem only occurs if a workbook contains multiple worksheets and at least one of them (except the last one) doesn't contain comments.

Example:
Assuming a workbook has 3 worksheets, there will be 3 files in \xl\worksheets.
sheet1.xml (sheet without comments), sheet2.xml and sheet3.xml.
Since the first sheet doesn't contain any comments, there will only be two comment files in \xl\.
comments1.xml, comments2.xml.

Currently the relation between sheet<number>.xml and comments<number>.xml seems to be index based, according to Comment.go.

Code snippet from link above:
commentID := f.GetSheetIndex(n)
commentsXML := "xl/comments" + strconv.Itoa(commentID) + ".xml"

Solution:
Matching sheet<number>.xml and comments<number>.xml based on a common index returns wrong matchings, if any worksheet (except the last one) has no comments.
It seems that the proper relationship between worksheets and comments can be found in \xl\worksheets\_rels.

This folder contains XML files for each worksheet that has any relationships. A plain worksheet will have no file in \xl\worksheets\_rels.

According to the example I mentioned above,
sheet1.xml.rels contains the lines

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
                <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/table" Target="../tables/table1.xml"/>
</Relationships>

Note that there is no reference to \xl\comments1.xml.

sheet2.xml.rels contains the lines

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
                <Relationship Id="rId2" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments" Target="../comments1.xml"/>
                <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing" Target="../drawings/vmlDrawing1.vml"/>
</Relationships>

Note that there is a reference to \xl\comments1.xml in line

<Relationship Id="rId2" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments" Target="../comments1.xml"/>

To avoid the issue, I would propose to match worksheets and comments based on their relationship in
\xl\worksheets\_rels. Please note that a plain worksheet will not have a corresponding file in \xl\worksheets\_rels. Thus it may happen that you will only see sheet2.xml.rels and sheet3.xml.rels, assuming that worksheet one is plain, but worksheet two and three contain comments.

Steps to reproduce the issue:

1. Create a new Excel file
2. In case you don't have three worksheets by default, create two additional sheets.
3. Set a comment in any cell on worksheet 2 and worksheet 3. Don't set any comment on worksheet 1!
4. Call GetComments()

Describe the results you received:
Output with some additional formatting:

Sheet1 [{xxx, xxx 0 A1 This} {xxx, xxx 0 B1 Is} {xxx, xxx 0 C1 A test comment}]
Sheet2 [{xxx, xxx 0 A1 Another} {xxx, xxx 0 B1 Test} {xxx, xxx 0 C1 Comment}]
Sheet3 []

Describe the results you expected:
Since the first sheet had no comments, but only sheet two and three, I would expect:

Sheet1 []
Sheet2 [{xxx, xxx 0 A1 This} {xxx, xxx 0 B1 Is} {xxx, xxx 0 C1 A test comment}]
Sheet3 [{xxx, xxx 0 A1 Another} {xxx, xxx 0 B1 Test} {xxx, xxx 0 C1 Comment}]

Output of go version:

go version go1.11.5 darwin/amd64

Excelize version or commit ID:

v1.4.1

Environment details (OS, Microsoft Excel™ version, physical, etc.):
OSX Mojave 10.14.3
Microsoft Excel 2010