Use DT row selection instead of checkboxes

[nuno-agostinho]

DT tables allow to select multiple rows in different pages unlike the current implementation with checkboxes. Unfortunately, the current version of DT doesn't return the indexes of the selected rows when using the Bootstrap style in the version released in CRAN, although this was already fixed in the development version.

As soon as this version is available in CRAN, the implementation will be changed.

[nuno-agostinho]

Check if #193 resolves the issue.