Create a robot pool

[jbmouret]

This is useful when we use robot_dart in many threads and we want to avoid parsing new robots from scratch.

Fix #99

[costashatz]

@jbmouret thanks for this. I pushed a fix for a missing linking flag, but I have one bigger concern. What happens if we want to use to use two robots (and thus there is a need for 2 robot pools with different robots)? The way it is now, this is not possible..

[jbmouret]

Good question:

• we can choose to not cover this case
• we can suppress the singleton access (it is handy but the user can do a global variable instead or link instances)

I would vote for the second case.

[costashatz]

I would vote for the second case.

Agree!

[costashatz]

Merging...