Suggest a value for frames_per_block if the memory is insufficient

[ptim0626]

This PR aims to provide a conservative value for the parameter frames_per_block if the block cannot be fitted into the current GPU memory.

It is more helpful for PtyPy to provide a value for frames_per_block as most of the users do not have any idea what a sensible value should be. The ideal way is to provide an option such as frames_per_block='auto' so the user can forget about it but the memory estimation happens after the data is loaded alongside with other stuff and it also depends on the algorithm. As a result it is not possible to achieve this as it is wished here.

When the block is too big, it prints out a message similar to this:

Cannot fit memory into device, if possible reduce frames per block. Exiting...
Your current 'frames_per_block' is 2000.
With current reconstruction parameters and computing resources, you can try setting 'frames_per_block' to 721.
This would divide your reonstruction into 3 blocks.
ptypy has been exited.

This is only relevant for any _Block model.

This also fixes a small bug by removing self.context in the CuPy's EPIE engine.