Added NCM replace dictionary and used in icarl

[AndreaCossu]

Now NCM allows to replace all the means. This is useful in iCarl, where means are replaced after every experience.

[coveralls]

Pull Request Test Coverage Report for Build 5324381347

• 14 of 14 (100.0%) changed or added relevant lines in 3 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.001%) to 72.689%

---

Files with Coverage Reduction	New Missed Lines	%
avalanche/models/ncm_classifier.py	3	89.58%

Totals
Change from base Build 5287812284:	0.001%
Covered Lines:	16334
Relevant Lines:	22471

---

💛 - Coveralls