Deepreg: Demo optimise - paired mrus brain

[s-sd]

Subject of the issue

Improve the results from the paired_mrus_brain demo.

[YipengHu]

we can leave this ticket open, but i would not spend too much time on this one now.

[mathpluscode]

Hi, please do not continue this issue before the merge of PR #602 as this PR will change network structures.

[YipengHu]

i'm closing this in favour of a new ticket with better-defined objective, given a updated demo already in place.