Resolve "Add a validation wrapper for "vagrant up""

The amount of memory and cpus included in the intermediate definition is now compared to the momentary available memory and number of cores (threads) on the host system before build. Free disk space is also checked and compared to a rough estimation of space needed by the sandbox (it is difficult to get a precise value). A warning is displayed when some of the checks fail, but the building process continues. Closes #25 (closed)