Print a warning message when fewer than 40 bridge samples in cross-product bridging

[klevdiamanti]

Title: Print a warning message when fewer than 40 bridge samples in cross-product bridging

Problem: In case of fewer bridge samples than required, cross-product normalization reports NA in QSNormalizedNPX but no warning message.

Solution: Print a warning message for the case above.

Checklist

• Make sure you are requesting to pull a feature/bugfix branch (right side). This should not be main or develop.
• Make sure you are make a pull request against either main or develop (left side). (Requesting to main should be reserved for bugfixes and new releases)
• Add or update unit tests (if applicable)
• Check your code with any unit tests (Run devtools::check() locally)
• Add neccessary documentation (if applicable)

Type of changes

What type of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue) (link the issue on the right)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code style update (formatting, renaming)
• Documentation Update
• Other (explain)

Further comments

Addresses #508.