separate APIs for initializing and enumerating sysman

[bashbaug]

This PR has several changes related to #7:

• It adds a zesInit function to initialize sysman separate from the zeInit function. It does not require an environment variable.
• It adds separate zesDriverGet and zesDeviceGet functions to enumerate sysman drivers and devices, rather than enumerating devices via core APIs and casting to sysman device handles.
• After enumerating device handles, sysman can be used exactly as it was before.

I added the APIs and I also updated the programming guide to describe how they are used.

Things we still need to decide (could be fixed in this PR or separately):

• Do we want query functions to determine if a core driver or device handle can be converted to a sysman driver or device handle, e.g. zesDriverSupported, or are the separate functions to enumerate sysman drivers and devices sufficient?
• Likewise, do we want a function to convert between a core driver or device handle to a sysman driver or device handle?