Add batch updates for Gramians

[simon-hirsch]

This PR adds the possibility to batch-update the gramian matrices to the package. This is the first step towards proper (mini-) batch updates in our package.

This PR additionally adds a whole lot of testing around the Gramian matrices, including verifying that

• plain single step updates work as expected
• batch updates work as expected

for

• multiple dimensions
• multiple forgets
• constant and random weights

[BerriJ]

This looks good. Thanks for directly adding tests.