Fix problem with unknown datatypes in replay/tracing.
When datatype was unknown to replay, it was replayed as MPI_BYTE.
This modification adds a parameter to encode_datatype, to tell tracing that the datatype size has to be taken into account in the count parameter
This results in the fact that a message of count*datatype_size being replayed as a message of (count*datatype_size)*sizeof(MPI_BYTE), which is the same.
This is not a perfect or elegant solution, but :
- it works.
- it handles manually created datatypes
- it doesn't break previously generated replay files
- it avoids testing each time 50 different datatypes (see encode_datatype function)
- the new parameter avoids doing strcmp with "-1" at each time, performance should not be too bad
