Remove some TODO comments

We use the issue tracker for TODOs
rename build-tools dir to tools
boost: cleanup, reduce build time
gap/pkg/common.mk: allow custom fetch command
Update .gitignore
singular: workaround for issue with official Singular 4.0.1 tarball
flint: do not build static libflint.a
add ntl package
singular: build with ntl support
Add GAP package "SingularInterface"

Closes #6
Add GAP package "NormalizInterface"

However, it is disabled for now, as it requires a not-yet-released
version of normaliz.

Closes #7
Add GAP package "ferret"