[utils] check for input file extension

[rohanpsingh]

Throw error if argv[1] does not have a .bin extension

[gergondet]

That's not super necessary, the FlatLog class checks that the log file is a valid mc_rtc log by checking for a magic number at the start of the file. What was the motivation for this?

[rohanpsingh]

In that case we could probably put the check early on. Currently, even if the input file is invalid, the program will generate an empty output file and then fail after.
I agree it's not super important.

[gergondet]

I see, I think the nicer approach is to:

• check the log size after opening it and before creating the new file https://github.com/jrl-umi3218/mc_rtc/blob/master/utils/mc_bin_to_flat.cpp#L364
• maybe return a bool/int from mc_bin_to_flat so that the util can return a different exit code if the conversion fails
• do the same with mc_bin_to_log and mc_bin_to_rosbag (https://github.com/jrl-umi3218/mc_rtc/blob/master/plugins/ROS/utils/mc_bin_to_rosbag.cpp)