Regression: Exception when creating VUnit instance when no simulator is installed

[joshrsmith]

Checking for the simulator being installed should be delayed until the simulator is actually needed. I am guessing that there is some coupling between detection of what simulator is available and the UI/arguments that are supported relating to that simulator.

For example, if you simply wanted to create a VUnit project: prj = VUnit.from_argv() -- why do you need to have a supported simulator installed?

I believe there is a regression introduced in cc534bd. This issue was originally fixed in 4bbd629, related to #67

Similarly -- It might be nice to also have a command line argument which can ignore that a supported simulator has not been detected.