models with the same name in different directories will not be distinguished

[pkienzle]

The function generate.tag_source returns a unique string associated with the source code for the model. Use this rather than time stamps to determine if the model needs to be recompiled. Remove all the time stamp comparison code since it is complicated and error prone.

Precompiled models that are supplied with the windows installer will need to include the tag, which may complicate the installer. Are precompiled models needed given that SasView ships with tinycc?

OpenCL kernels already use the tag for testing if a recompile is needed. It also check timestamps, so kernelcl.py should also be updated.

In practice, this issue will only be a problem for users that don't have OpenCL but are making their own versions of models with the same name as the builtin model, and still want to run the builtin model. Currently, the most recently changed source will win.