Cmake version config

Now, projects that depend on Simbody can use a line like the following:

```
find_package(Simbody 3.4 REQUIRED)
```

See my toy project https://github.com/chrisdembia/sample-simbody-client/blob/master/CMakeLists.txt

A requested version is compatible with the installed Simbody version if they have the same major and minor version, and the requested patch version is less than or equal to the installed version's patch version.

If, instead, we want instead for compatible versions to be those that have the same major version , and the requested minor version is less than or equal tot he installed version's minor version, that is easy to do. Just let me know.

Here's something from CMake's documentation:

```
  if(version1 VERSION_LESS version2)
  if(version1 VERSION_EQUAL version2)
  if(version1 VERSION_GREATER version2)
# Component-wise integer version number comparison (version format is major[.minor[.patch[.tweak]]]).
```