MAINT: Refactoring the way nipype checks Node hashes

[oesteban]

Hopefully, to make it a bit simpler. This could affect #2380.

Main ideas:

• Add a new Node.is_cached() member, trying to remove the responsibility of checking hashes from teh Plugins and putting it all in the Node context.
• Simplify Node.hash_value() with the use of is_cached().
• Node.needed_outputs is now a property (allowing some checks when setting).
• Coordinate get_inputs, hash calculation, needed_outputs calculation and the hash elements: _hashvalue and _hashed_inputs

[oesteban]

Maybe not necessary for this release, if you guys are very busy (as I imagine), but this should simplify hashing nodes. It'd be great to hear from @satra on this.

[effigies]

I don't feel capable of reviewing this before 1.0. @mgxd @satra if you do, please tag it with the 1.0 milestone.

[satra]

@oesteban - this looks good to me.

[satra]

@effigies - your call if you want to include this in 1.0. it will make the code look a lot cleaner.

[satra]

@effigies - it would be fine with me to include this.

[effigies]

No objections from me, I just haven't reviewed it, myself. Feel free to merge.