Pull all of the apps in the scripts directory into a single command line interface

[mfitz]

GeNet has a bunch of useful python applications in the scripts directory to perform tasks such as validating or simplifying a network, or adding elevation and slope data to a network. It would be good to pull all of these different apps into a single, discoverable CLI so that we could issue commands such as genet network simplify, genet network validate or genet network add-link-slopes. The CLI should be discoverable, enabling commands such as genet --help, genet network --help or genet network simplify --help.

[KasiaKoz]

We need to make sure that the recent summary stats: #135 are included in the cli

[KasiaKoz]

Done in #194