NGS toolkit should provide an API to set cache configuration options

[pditommaso]

Currently when accessing read collections by using the NGS API download reads are cached in the path $HOME/ncbi/public for open-access data, and the path $HOME/ncbi/dbGaP-xxxx for other.

There's no way to change these paths in the current API. It would be useful to have the ability to set a custom path for cached reads, including the possibility to disable the caching feature at all.