Small molecules added from NIST-type library are missing ExplicitRetentionTime

[meowcat]

Steps to reproduce:

1. create blank document in molecule mode
2. View -> spectral libraries. Dropdown -> Add, choose a name, add the MSP file from this gist:
   https://gist.github.com/meowcat/4f0a08cd803d941cd1ac53829931ee5f
3. Select the new library in the dropdown, select the "Metamitron" molecule, "Add"
4. Confirm "adding the library to your document"
5. Right-click on the added entry "Metamitron...", modify
   Explicit Retention Time is missing.

Investigations:

• RetentionTime is parsed correctly from the MSP.
• When performing AddPeptide:

  pwiz/pwiz_tools/Skyline/SettingsUI/ViewLibraryDlg.cs

  Line 1734 in fd07aa9

  public void AddPeptide(bool addLibraryToDocSeparately)

  At L1784 breakpoint, in pepMatcher.PeptideMatches[0].Value, the retention time is present in LibInfo.RT but absent in NodePep.ExplicitRetentionTime.
• I haven't figured out yet where the PeptideDocNode entries are created from (supposedly) the library info.

[meowcat]

Note: this was similarly reported in 2019, but was reportedly addressed: https://skyline.ms/announcements/home/support/thread.view?rowId=38820

[meowcat]

In

pwiz/pwiz_tools/Skyline/Model/LibKeyModificationMatcher.cs

Line 339 in 1f52dda

var nodePep = CreateDocNodeFromSettings(key, peptide, diff, out nodeGroup);

, the nodePep created from CreateDocNodeFromSettings is missing the ExplicitRetentionTime

[meowcat]

I have a fix in #2860 but I don't know if that's the right way to do it.