Ncbi update option

[JohnBioinf]

I ran into problems using the NCBITaxa class.
Initiating the class always triggers a check if the current data base is not up to date. This causes other processes to throw sqlite errors when they want to access the database at the same time.

Therefore I added an option on initializing the class to not update the database. The default in this pull request still is that an automatic update is triggered, hence this pull request is down stream compatible. If any software relies on the automatic update on class initialization.

I changed the official documentation to clarify the way how the data base can be updated. And further added a few explanation on some option regarding the initialization of the class and a formatting error.

Currently I need some try and except statement to catch the sqlite errors in my own project. This could be avoided with this pull request.

If you find this helpful I would be glad if you merge it.

Thanks
John