selectsites failing with poor message on unsorted sites processed on disk

[andrewdavidsmith]

Describe the bug
When the sites are not pre-loaded they need to have a very specific order on the chromosomes and this is not made clear to the user. In addition, the message about forcing pre-laoded sites when the input is compressed is not sensible.

To Reproduce
Steps to reproduce the behavior:
Do selectsites with an order on chromosomes that puts, e.g., chr10 before chr1. This will fail without a reasonable error message unless the sites are pre-loaded.

Expected behavior
And error message should indicate the required order when sites are processed on disk. Better: the order of sites may be determined by a quick scan of the input file to find the chromosome order, so that the only sorting strictly required would be within chromosomes.

Desktop (please complete the following information):

• OS: Ubuntu 22.
• Version: 1.3.0