[HEADLESS] Get default JVM memory from Gama.ini

[RoiArthurB]

Is your request related to a problem? Please describe.
As pointed in the issue https://github.com/gama-platform/gama/issues/3658#issuecomment-1477868439 it can be a little confusing to have to everytime manually set the memory amount in GAMA headless command if the user did already set it in GUI mode (or manually in Gama.ini).

Describe the improvement you'd like
Make the gama-headless wrapper read the Gama.ini file and get the -Xmx value by default if no memory is set by the command (flag -m). Moreover, atm both sides are set on 4Go of RAM, which wouldn't cause any major modification to the headless script.