Small update of the Search module that makes the code a bit more portable.

This is an update of the Search module. The static members of the Selector class, e.g. mass, PDG id selectors are moved to a separate class StandardSelector inherited from Selector. The first reason is that the Selector has recursive relations with the SelectorWrapper class which makes it VERY hard to separate into a self-consistent library and header. The later is important on Windows (and Mac?). Particularly this separation has allowed the building of the corresponding Python3.8 interface on Windows. The second reason is that this looks like a better design and inheritance structure. The basic class Selector stays untouched and the StandardSelector can be expanded with more static functions.

The update is small but it would be nice if @lonnblad would say OK.

Andrii