Add ability to query number of internal ciphertexts

[sloede]

I wonder if we should add something like nciphertexts(sa::SecureArray) or nvectors (or maybe some better name that I cannot come up with) to return the number of ciphertexts used to encode a given SecureArray. It might be nice to have this to estimate whether it makes sense to use multithreading in Julia or based on OpenMP.

We probably would want also nciphertexts for the Unencrypted backend...