Add the ability to generate an average speed curve for a given user's previous activities

[FJBDev]

For a given distance, a user might want to know what is his/her average speed considering multiple outings. Example :

• A user would like to generate his/her average speed for the 50km distance
• The user selects a folder where a list of files for this distance are present.
• Course Generator reads all those files and generate a curve for which the speed/slope is that average of all those files

[patrovite]

Generating a speed/slope curve from a set of track is in my todo list since a long time. I never took the time to implement it. The "funny" thing is that it's not very complicated to implement ;)