add rand rot90 transforms and dictionary-based counterparts

[wyli]

relevant issues #72 #58
fixes #59 fixes #63

Description

adds

• rotate90 transform
• randomised rotate90 transform

in vanilla and dictionary-based formats.

organises:

• vanilla transforms (callables that deal with image array) in monai/transforms/transforms.py
• dictionary transforms (callables that deal with data dict) in monai/transforms/composables.py
• randomizable interface for random augmentation transforms

could you have a look?
@atbenmurray @ericspod @Nic-Ma @yanchengnv
it could be a starting point for further discussions

Status

READY

Types of changes

• Breaking change (fix or new feature that would cause existing functionality to change)
• New tests added to cover the changes
• Docstrings/Documentation updated

[Nic-Ma]

Added comments inline.
Thanks.

[Nic-Ma]

Added comments inline.
Thanks.

[wyli]

before a proper adaptor is in place I reckon that we go for dictionary-based wrappers around each vanilla transform. this PR's Rotate90d and RandRotate90 are dictionary-based. users could also choose to use the vanilla directly or build customised adaptors around them. @atbenmurray @ericspod

[ericspod]

Very minor changes but otherwise

[wyli]

thanks for the online/offline discussions, @ericspod @atbenmurray @Nic-Ma @yanchengnv , I think we are making good progress here.

notebly, this PR have organised

• vanilla transforms (callables that deal with image array) in monai/transforms/transforms.py
• dictionary transforms (callables that deal with data dict) in monai/transforms/composables.py
• randomizable interface for random augmentation transforms

I'll update the unit tests and merge (still open to any comments/suggestions if any)