Streamline private cluster setup in client

[jpswinski]

Currently, the call to icesat2.init only supports private clusters that are already running and where the user has their username and password in a .netrc file or in an environment variable. The following does not work:

• If the user wants to supply their username and password within the script, the init call will throw an exception because the version check will completely fail.
• If the cluster is not running because it is configured to be fully destroyed when not in use, then the init call will throw an exception because the version check will completely fail.

The icesat2.init function should support passing through the username and password AND should also perform an aliveness check on the cluster, and if the cluster is not up - it should "wake" the cluster up automatically, using a parameter that defaults to one for the number of nodes.

[jpswinski]

f91540b - updates to init function for desired nodes

[jpswinski]

We are not going to pass through the username and password because we want to discourage this way of providing credentials in favor of the netrc file or environment variables.