specify required disk space for packages and checking such space

[gfursin]

@SerailHydra suggested a possible good extension to specify required disk space for very large packages (such as ImageNet training data sets). We can just add an extra tag "required_space" and if it's !=None, we can check space and add a warning if space is lower ...

[gfursin]

We should also make it portable (i.e. on Linux and Windows) ... I hope there is a way in Python to get free space on a disk by the installation directory ...