Add a SV_BUILD_DATE option to set the install date

[ktbolt]

The plugin CMake currently creates its install directory using the SimVascular build date it builds from and the date the plugin itself is built. This may produce a plugin install date different from the current SimTK SimVascular install date which will cause the plugin not to load.

To set a build date different from the SimVascular build date add a SV_BUILD_DATE variable to CMake that can be set from the command line like this

cmake .. -DSV_BUILD_DATE=2022-07-20  -DACCEPT_DOWNLOAD_EXTERNALS=ON