Add toolchains for LCG nightlies
The most flexible way to use LCG nightlies via lcg-toolchains is through the special toolchain file special/lcg-nightly.cmake, but that is very unpractical for end users, who would prefer to do something like LCG_VERSION=dev4 as they do LCG_VERSION=100.
With this MR I add a bunch of toolchains (ultimately delegating to special/lcg-nightly.cmake) that can be found specifying as LCG_VERSION strings like:
dev4dev4/Todaydev4/Wed
where dev<X> and dev<X>/Today are equivalent and both delegate to the specific dev<X>/<day>.
With this approach one is required to create the specific files before using a new platform for LCG nightlies.
See LBCORE-2032
Edited by Marco Clemencic