Print the range including 95% of the hash rate values

[marktwtn]

Implement functions of getting the average value and standard deviation.
Print the hash rate range when the BUILD_STAT option is enabled.
Close #77.

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The output message with BUILD_STAT option enabled would look like this:

Hash rate average value: XXXX.XXX kH/sec
with the range +- xxx.xxx kH/sec including 95% of the hash rate values.

[furuame]

Here is my suggestion:

1. How about executing PoW POW_TOTAL times only when ENABLE_STAT enabled? We just need to check correctness unless ENABLE_STATE enabled.
2. I'm not sure whether executing the PoW for the "same" transaction tryte many times achieve the original goal. Or, you can also consider doing PoW for different transaction trytes. There are some testing data in tests/trytes.txt

[marktwtn]

1. How about executing PoW `POW_TOTAL` times only when `ENABLE_STAT` enabled?
We just need to check correctness unless `ENABLE_STATE` enabled.

I agree. Just make it simple.
If ENABLE_STATE is disabled, check the correctness.
If ENABLE_STATE is enabled, check the correctness and do the hash rate statistics analysis.

The program structure would be the same, except that there would be a variable which controls the total execution times of PoW.
If ENABLE_STATE is disabled, the total execution times equals to 1.
If ENABLE_STATE is enabled, the total execution times equals to macro POW_TOTAL.

2. I'm not sure whether executing the PoW for the "same" transaction tryte many times
achieve the original goal.
Or, you can also consider doing PoW for different transaction trytes.
There are some testing data in `tests/trytes.txt`

I guess it still achieves the original goal since we only care about the hash rate, which is dependent to the implementation?

[furuame]

Don't you care the elapsed time of each PoW?

[marktwtn]

We only focus on the hash rate now.
If the elapsed time of PoW of different transaction tryte needs to be shown, I will make sure each execution of PoW uses the different transaction tryte.