Support multiple devices and contexts

[jcuda]

The current handling of devices and contexts in the jcuda-vec classes is very limited. The library can attach to a single context, and only to one context. If no context is available, it will create one, but always for a single device (the first one).

In order to improve this, serval options have to be considered:

• whether there has to be some cuCtxSetCurrent call inside of the VecFloat methods
• whether it will be the responsibility of the user to call something like VecFloat.attachToCurrentThread() before using the VecFloat class on a different thread
• whether it will be the responsibility of the user to call something like VecFloat.setCurrentContext(myContext) before using the VecFloat class on data that belongs to myContext
• whether one VecFloat instance has to be created for each context (because the modules that are created during the initialization of the VecFloat class belong to one context only)
• whether the VecFloat will need a (context-) "handle", like it is used in the newer versions of the runtime libraries, and which has to be passed to all method calls

( Already listed in this forum thread (the formatting is a bit broken, due to a forum software update - sorry))