Add support for importing more than just the "Personal Best" comparison

[amyy54]

The only function for importing a comparison, "import_comparison", would only ever read the "Personal Best" comparison from the file when importing. This commit adds the "import_comparison_as_comparison" function, which allows for a named comparison from the file to be specified, instead of just loading "Personal Best".

Due to this addition, "import_comparison" is reduced to calling "import_comparison_as_comparison", with the name specified directly as "Personal Best".