Set default number of threads to 1

[marcelm]

Also, adjusted logging output to make it more obvious how many threads are used (and which mode). Example:

...
Total time indexing: 3.79 s
Processing paired-end reads in mapping-only mode using 1 worker thread
 Mapped         1.00 M reads @    11.02 us/read
Done!

Other example messages:

Processing single-end reads in abundance estimation mode using 4 worker threads
...
Processing single-end reads using 4 worker threads

Closes #401

[ksahlin]

Great! About your comment on making it 'easily interpretable' for less experienced users (#401); what do you think about removing 'worker' in 1 worker thread and just state using 1 thread?

One may think that worker threads are not directly related to number of threads the program is using. For example, in the producer/consumer solution there is one producer and several consumers that sometimes might be confused with 'workers'.

[marcelm]

One may think that worker threads are not directly related to number of threads the program is using.

I somehow misremembered that the main thread does the I/O, so it was actually my intention to convey that there are more than $n$ threads, but checking the code, I see that that isn’t the case: The main thread only prints the progress indicator, so it shouldn’t be counted. I have changed the text as you suggested.

[ksahlin]

Great, approved!