time utils

[ipapandinas]

The following blockchain utils functions are needed:

• [x] blockToTimestamp (ex : block 1000000 => 1st of january)
• [x] timestampToBlock (ex : 1st of january => block 1000000)
• [x] blockToDuration (ex : 10 blocks => 60000 miliseconds)
• [x] durationToBlock (ex : 60000 miliseconds => 10 blocks)

[ipapandinas]

These utils were implemented here: https://github.com/capsule-corp-ternoa/ternoa-js/commit/36c803fba549f6c1b179dbb7c7fd75b91670b8f5

The future date prediction has some slippage.