Fixed some warnings.
Fixed a bug in the POVray exporter. It happened that, if one removed a ChBody from the simulation while running, the POVray exporter still had a list of cached assets and some of them were not used anymore.
The new implementation scans the hashtable of cached assets to see if some asset is referenced only by the hashtable, using the ref counter of the shared pointer, and if so, it can remove it safely from the hashtable.
IMPORTANT. Change in API of shared pointers. Previous code will work as before, but in some cases you will need to add  "DynamicCastTo<...>()" when casting shared pointers.
Background---
Previously, casting between ChSharedPtr shared pointers was too 'liberal', in the sense that it also allowed casting bwtween unrelated pointer types.
Not only this was inconsistent with the way that C++handles normal pointers, but also this left room to programmer mistakes.
Solution---
In the new ChSharedPtr it happens that:
- casting between shared pointers is automatic if upcasting from a specialized class to a parent class, or between same classes, as in c++ pointers; in this case your code does not need any changes.
- casting between shared pointers in downcasting from a base class to a specialized class now explicitly REQUIRES A DYNAMIC CASTING, otherwise you get a compiler error.
- casting between unrelated pointers will generate a compiler error at compile time.
The dynamic casting can be obtained by using the new   DynamicCastTo<>()  function, or by using the new    dynamic_casting_chshared<>(...)   method that is very similar to c++ dynamic_casting.
This means that in this commit we had to change many parts of the code where a base shared pointer was previously cast to specialized classes by doing simply
   ChSharedPtr<MySpecialClass>myspecialized = mybase;
into the new:
   ChSharedPtr<MySpecialClass>myspecialized = mybase.DynamicCastTo<MySpecialClass>();
Note that this requires more typing, but it is less prone to misunderstandings than the previous approach.
The Python wrapper for generating the Chrono::PyEngine units have been modified according to the new safer API of the ChSharedPtr.