Release 3.4

Version 3.4 release.

• Updated tt_gcp_fg_setup to fix Negative Binomial loss function. Also added tests/Test_GCP_OPT.m. See merge request !78 (merged), #65 (closed).
• Updated tt_sample_zeros to use efficient ismembc. See merge request !77 (merged), #64 (closed).
• Fixed off-by-2 bug in gradient computed by @ktensor/fg.m.
• Described scale option in cp_opt.
• Minor spelling corrections in documentation.