Write PRV file in blocks, show progress

Currently writing the PRV file can be very slow for a large dataset. The writing is done in two phases, first putting all events ascii-encoded in a variable, then writing to file. Also, there is no progress reporting with this implementation.

Some improvement suggestions are:

• Fill variable in smaller size chunks/blocks and write after
• Pipeline the encoding in ascii and the writing to file with multithreading
• Compute a progress percentage based on number of blocks to be written
• Find more system-aware ways to encode the events into ascii and then write