--interact is not working about vowpal_wabbit

[ataymano]

1. It only works on dense namespaces with first feature has to be equal to 1 (from here)
2. explicit 0 values are getting ignored by parser
   So, dataset should be changed to (in order to demonstrate happy path):

0.24 |a 0:1 1:0.88 |b 0:1 1:0.28
0.06 |a 0:1 1:0.46 |b 0:1 1:0.14
...

[ataymano]

Tried to cleanup anchor features here but cancelled PR (#4646) after conversation with @jackgerrits: not having anchor features is breaking offsets calculations if first feature turned out to be 0.
Leaving unchanged for now but should be used very carefully.
Counter-intuitive effects with current implementation:

1. Working on feature group level, so having a lot of unexpected behavior when more than one namespace with same first letter exists.
2. Destination hash is calculated based on first namespace (from commandline) => non-commutative, "--interact ab" and "--interact ba" are producing different hashes
3. Destination hashes non sensitive to hash shift of second argument:
   for "--interact ab" configuration:
   "|a 0:1 1:1 |b 0:1 1:1" is producing the same hashes as "|a 0:1 1:1 |b 3:1 4:1", but dufferent from "|a 3:1 4:1 |b 0:1 1:1

[ataymano]

wiki page is created:
https://github.com/VowpalWabbit/vowpal_wabbit/wiki/Element%E2%80%90wise-interaction