Use of PAAL by the Wire‐Cell toolkit

WCT requires PAAL for the code that is ported from the WC prototype such as BNLIF/wire-cell-pid. PAAL use in the prototype was via a package BNLIF/wire-cell-paal. This packages includes the PAAL headers in a flattened directory tree.

Unlike the prototype, WCT relies on PAAL installed "for real" and in particular in its intended include/paal/ tree structure.

Thus, it is up to the user to install PAAL. However, wire-cell-spack installs PAAL when the +paal build variant is given. Note, this install does NOT build PAAL binaries nor tests as PAAL suffers some bitrot due to the non-standard package "numeric bindings" not ever being added to Boost and now apparently abandoned. Use of the portions of PAAL that depend on this missing functionality will certainly lead to breakage.