Change abagen.mouse data fetcher caching location

[rmarkello]

The issue

If you use any of the data fetchers in the abagen.mouse module the results are cached in the installed abagen distribution. Unfortunately, if you install a new version of abagen (e.g., via pip install --upgrade abagen) that distribution is wiped, so the next time the data are fetched they'll need to be re-downloaded from the internet which can be costly / annoying.

Proposed solution

Cache the data in a different location! This is highly related to #44; it might make a lot more sense to use a centralized location for all the abagen data fetched from the internet.

[rmarkello]

Closed with #179 .