Some Python3 and multi-robot fixes

[marc-hanheide]

Apologies conflating two bugfixes into on PR, but it's simple (and late say @gcielniak):

1. the map converter didn't work in noetic
2. some topic names where "root-ed" that shouldn't be in multi-robot (namespace) setups

[marc-hanheide]

retest this please

[marc-hanheide]

@Jailander can you briefly check, as I'm unsure if the / in those topic names was there for a reason?

[Jailander]

@adambinch can you help me with this I have my hands full this week

[marc-hanheide]

Many thanks, I'll merge