Add StateSaver class.

[brianhou]

RAII class suggested by @mkoval in personalrobotics/libherb#26.

[coveralls]

Coverage decreased (-0.02%) to 80.35% when pulling 2068061 on enhancement/brianhou/state-saver into 2af5c55 on master.

[coveralls]

Coverage decreased (-0.02%) to 80.35% when pulling 2068061 on enhancement/brianhou/state-saver into 2af5c55 on master.

[mkoval]

This looks great. Nice work @gilwoolee.

I filed #185 as a follow-up issue to revisit the implementation once we understand what other properties need to be preserved.

We also need to decide how to handle topological changes in a MetaSkeleton (e.g. change in the number of DegreeOfFreedoms) that make this class semantically meaningless. That is a broader issue in DART, so let's ignore it for now.

[coveralls]

Coverage decreased (-0.01%) to 79.153% when pulling c6d21d7 on enhancement/brianhou/state-saver into 5d19317 on master.

[jslee02]

Nit: It would be nice if we have a simple validity check whether the size of mPositions is still equal to the dofs of the MetaSkeleton (if not, we print a warning). I understand it wouldn't be able to catch other structural changes such as changing the joint type with the same dofs, but at least we could catch dimension changing cases.

[coveralls]

Coverage remained the same at 78.813% when pulling e5e1b8d on enhancement/brianhou/state-saver into 4c08ac5 on master.

[coveralls]

Changes Unknown when pulling 412feea on enhancement/brianhou/state-saver into ** on master**.

[coveralls]

Changes Unknown when pulling 412feea on enhancement/brianhou/state-saver into ** on master**.

[coveralls]

Coverage remained the same at 78.813% when pulling 412feea on enhancement/brianhou/state-saver into a806042 on master.