.n_occupied() was implemented differently in hashbits and hashtable

[qingpeng]

as discussed in #429 and #491

Also,in load-graph.py, '--report-total-kmers' option will print .n_occupied() as "Total number of k-mers".
In .info file, .n_unique_kmers() is printed as number of "unique k-mers".
This is confusing. I will look into this.

[mr-c]

Thanks @qingpeng

[mr-c]

Any progress?

[mr-c]

Upon further review I see that we only count unique k-mers in hashbits. n_occupied merely tells you the number of bins occupied in the first table.

I am going to implement a CountingHash.n_kmers() function and update the scripts to use it instead of n_occupied().