control number of decimals in prices returned by oracle

[tenmoves]

Description

since we added multiple sources for the returned prices in the oracle, different source return different number of decimals however this number is fixed depending on the currency.

see => http://www.londonfx.co.uk/ccylist.html and iso 4217

this impacts the bridge and the wallet.

Fixes #842

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules