Factorization object only return unique factors. Fixes #210.

[yhtang]

This PR also involves the following changes as by-products in order to address bug #210:

• redesigned the vectorize and view API.
• Factorization.__init__ now only takes in the incoming tsrex in verbatim and does not carry any postprocessing.
  • Since the method is never meant for end-users, a secret key ('50A-2117') must now be passed in order to use this method.

[campsd]

Looks good. I like the secret key 😄

One thing that I noticed, append was replaced by vec_axis in some but not all places. Any particular reason for having two different names?

[yhtang]

One thing that I noticed, append was replaced by vec_axis in some but not all places. Any particular reason for having two different names?

Well, it was when I was using it that I realize the append argument, without a proper context, may seem unintuitive for users. Hence, I renamed the user-facing arguments to vec_axis, but kept all internal usage as append. What do you think?

[yhtang]

Also, together with renaming nvec to vec_size, we unify the two related arguments so that they look more like a group: nvec -> vec_size append -> vec_axis

[campsd]

Makes sense!