Split code into Core & Legacy code (was ACTS-434)

Original author Andreas Salzburger @asalzbur

As discussed previously, we would like to split the acts-core repository into a Core and Legacy structure, in order to allow building up a pure Core set while still have the Legacy code to compare.

Letter can be added by defining a CMake flag.

Edited Mar 27, 2019 by Moritz Kiehn

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