Adding L2 regularization and other miscellaneous functions about slideflow

[jamesdolezal]

Sara - overall very good implementations! I've gone through the branch and made some changes. Here's a quick rundown of the changes I made in my two commits on your branch:

Commit 8a6e33f:

• Good work on this function! I mostly just added the ability to pass "None" as a destination option, which then allows the tiles to be extracted to the dataset's "tiles" directory as per project configuration

Commit 6a88b1d:

• Nice regularizer function! I moved it from the sf.util module into the sf.model module, for organization reasons
• Changed max_ and min_tiles_per_slide so that these criteria are applied to all TFRecords in the _interleave function
• Improved readability on some function calls. When function calls start getting really long, it's best to split them onto multiple lines. E.g. model.py lines 623-628
• Couple of code efficiency improvements. One example is lines 290-299; these could be condensed to just two lines (312 and 313)

I'll merge these changes into my dev branch shortly which has a number of other changes and improvements, but wanted a dedicated commit for your branch so you could see how I've implemented your code.