relax requirements of convertFromString

[v-lopez]

Don't fail compilation, send a warning instead

[facontidavide]

@mjeronimo for your information:

I decided to be less strict regarding convertFromString.

Now the program will compile even if the template specialization is missing.
If the NodeParameter points to a Blackboard, everything will work just fine since no string parsing is needed.

If either:

• NodeParameter was actually a string, not a pointer to a Blackboard entry or
• The Blackboard entry contains a string and you are trying to extract something else.

an exception is thrown, warning that you must implement your own convertFromString.

[mjeronimo]

@facontidavide Sounds reasonable. Thanks for the info.