More efficient Toolhandle<T>::get() method

I would like to use ToolHandle more, but I also want to use it optimally efficiently, by which I mean I want inline access to the underlying tool pointer, with no validity checks (I always `retrieve()` my tools up front in initialize()) and true const behaviour (for thread safety). Essentially what I want is `GaudiHandle<T>::get()`, but unfortunately in the current implementation this method is hidden behind the virtual `ToolHandle<T>::get()` method, that also down casts the tool to `IAlgTool*` (so no use for direct usage).

This MR fixes both these issues. get() is now fully optimal in `ToolHandle<T>` as it simply refers back to GaudiHandle in an inline method.

The `BaseToolHandle<T>::get()` method is just a wrapper that calls a hidden (protected) virtual function (`getAsIAlgTool()`) that returns what get() previously did.

I also tightened up the const'ness, providing both const and non-const methods that return const and non-const tool pointers respectively.

I also updated a few methods to remove multiple return statements (which normally cause compilers to abort inlining) and adding some `noexcept` here and there..

The only API change is that `ToolHandle<T>::get()` now returns `T*` instead of `IAlgTool*`, but this is not a problem as the user can still choose to use it as a `IAlgTool*` if they want..

Gaudi master builds fine with this, not tested any other projects yet.

cheers Chris

See merge request !218