User setting of NULLCOST parameter

[nicolaslhuillier]

Describe the current behavior

The NULLCOST parameter prevents using cost values lower than its value. Currently this parameter is set to 0.5 (in order to avoid having costs set to 0) and it is not possible to modify it.

Describe the expected behavior

It should be possible to modify the value of the NULLCOST parameter via the DSL configuration file.

Describe the motivation

Currently if a user wants to set negative costs, this is not possible. Moreover, if negative costs are set, these are silently replaced by the NULLCOST value (though the ADEQUAOF et REDISPOF parameters are meant to deal with negative costs). Therefore the default value of the NULLCOST parameter should probably rather be set to negative infinity.

Extra Information

This implies modifying metrix DSL and generation of fort.json. Modification of metrix executable should not be required as this already seems supported (to be confirmed)