NaN (not a number) position in Axis

(from @berruyer)

the position of a motor become Nan is a normal mode. Surely not an exception. In fact, the pb is that a number of motors may have, temporarily, and as a normal way of running, an invalid position, meaning that it cannot be calculated. The problem is how to represent it to the users and how to get this information in the code. For the moment the response is... Nan.

For the moment, this is the only way of working for a number of motors without breaking the functionalities of the others.

TODO:

• check NaN is accepted as a valid position
• ensure offset is not affected by position being NaN