Preprocessor update

— Adds support for “deterministic_trends”; as a consequence, run
  models/stochastic_trend_drift/trend1.mod to the testsuite
— The preprocessor now loads StatsFuns in its generated files, so add it to
  Project.toml (ref. #1)
1 job for master in 3 minutes and 22 seconds (queued for 5 seconds)
Status Name Job ID Coverage
failed unit_tests #28105


Name Stage Failure
unit_tests Test
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:72 [inlined]
[7] test(; name::Nothing, uuid::Nothing, version::Nothing, url::Nothing, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:89
[8] test()
@ Pkg.API /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:81
[9] top-level scope
@ none:1
Cleaning up file based variables
ERROR: Job failed: exit status 1