Merge branch 'master' of gitlab.epfl.ch:lamp/cs-210-functional-programming-2019 into week2-4

# Conflicts:
#	slides/sources/progfun1-2-3.md
#	slides/sources/progfun1-2-5.md
#	slides/sources/progfun1-2-6.md
#	slides/sources/progfun1-2-7.md
#	slides/sources/progfun1-3-1.md
#	slides/sources/progfun1-3-2.md
#	slides/sources/progfun1-3-3.md
#	slides/sources/progfun1-3-4.md
#	slides/sources/progfun1-3-5.md
#	slides/sources/progfun1-4-1.md
#	slides/sources/progfun1-4-2.md
#	slides/sources/progfun1-4-3.md
#	slides/sources/progfun1-4-4.md
#	slides/sources/progfun1-4-5-decomposition.md
#	slides/sources/progfun1-4-5.md
#	slides/sources/progfun1-4-6-pattern-matching.md
#	slides/sources/progfun1-4-6.md