add code to tower.{hpp,cpp} or use flint tower polynomials

Nobody has claimed this yet.

Some functions in tower.cpp have not been written yet. Something sensible needs to be done with them. In particular, the lift and promote need to be written before this ring type can become generally useful. On the other hand, maybe we want to instead use flint code for such ring elements (but this needs checking into).

I have removed the following warnings from the source code, instead we are documenting this need here.
```
** compiling ../../../../../../Macaulay2/e/tower.cpp
../../../../../../Macaulay2/e/tower.cpp:136:2: warning: "write Tower::is_unit" [-W#warnings]
#warning "write Tower::is_unit"
^
../../../../../../Macaulay2/e/tower.cpp:323:2: warning: "write Tower::promote" [-W#warnings]
#warning "write Tower::promote"
^
../../../../../../Macaulay2/e/tower.cpp:335:2: warning: "write Tower::lift" [-W#warnings]
#warning "write Tower::lift"
^
../../../../../../Macaulay2/e/tower.cpp:345:2: warning: "write Tower::syzygy" [-W#warnings]
#warning "write Tower::syzygy"
```

Start by reading tower.hpp and tower.cpp, focusing on the unimplemented Tower::is_unit, promote, lift, and syzygy functions. Investigate whether FLINT provides suitable tower-polynomial ring elements before choosing an implementation approach. Done means resolving that design choice and making the tower ring generally usable without the documented missing functionality.

Written by the indexing model from the issue text.