Issue #121: Move AssetDefinition and AssetDefinitionManager out of Wallet Model

    
    * refactored AssetDefinition and AssetDefinitionManager to use colormap instead of model
    * moved make_operational_tx_spec from AssetDefinition to BasicTxSpec
    * moved AssetDefinition and AssetDefinitionManager into ngcccbase/asset.py
    * added unit tests for asset.py in tests/test_asset.py