add Interface for incremental validating shareCount

[andreashappe]

Currently SecretSharing#reconstruct accepts a collection of shares and tries to reconstruct it.

For usage in the archistar-core prototype a new function "checkValidShareCount(Share[] shares)" would be needed. The method's implmentation differs between implemented algorithms. Shamir, Rabin, etc. would need at least k shares (archistar-core should be able to retrieve 2f+1 from BFT). When using RabinBenOr (or another robust secret-sharing scheme) this method would be called after each received share to check if valid share combinations were received.

[andreashappe]

Mostly this should be a helper class with methods like

• addShare -> adds a share to the helper instance
• canDecide -> are enough shares added to try to reconstruct secret