Add SSL and JSSL functionality

[georgeyiasemis]

This PR introduces the following:

• SSL MRI basic functionality (direct.ssl modules). This includes mask splitters to two (disjoint) sets in random uniform, random gaussian and half-mask manners.
• SSL MRI transforms: MRI transforms can now be SUPERVISED (as functionality before) and SSL_SSDU. The latter essentially splits the mask and k-space into two (disjoint) subsets (input and target).
• Added engines for SSL and JSSL (joint supervised and SSL)

[jonasteuwen]

Some small remarks.

[jonasteuwen]

LGTM