Support for software argument handler

[eblondel]

In some software, such as for DB connectors (#30), we need to be able to specify a handler function for a given argument.

For example, in the case of DBI, the main handler is DBI::dbConnect, but the first argument drv requires an instance of driver, that can be instantiated using DBI::dbDriver(<drivername>). The software argument definition could be extended with such handler, here in that case DBI::dbDriver.

The software function getHandlerInstance should then be able to evaluate such argument handler.

[eblondel]

Needed in #30