rosbag_storage: make Bag constructor explicit

[xqms]

This is a very small improvement to the rosbag API (adds "explicit" to the bag constructor)

Previously, it was possible to do something like this:

std::string bagFile = "...";
rosbag::Bag bag(bagFile);

rosbag::View view;
view.addQuery(bagFile, ...);

Note that bagFile is passed to View::addQuery, which does not cause a compiler error, since implicit conversion is allowed by the constructor. Using the view will cause a segfault, since the temporary bag will no longer be around.

I think that there are no good use cases for implicit std::string -> rosbag::Bag conversions, so we should disallow them.

[dirk-thomas]

Thank you for the patch.