fix: ensure dtcwt is not used in typing annotations

[mrava87]

The previous PR introduced an unwanted import error of pylops when dtcwt was not installed; this was due to the fact we used dtcwt for typing annotation.

The current solution is not to use for any typing annotation to ensure that no problem arises if dtcwt is not installed, given that it is an optional dependency

[cako]

For reference, one strategy is to check if the library providing the annotations is installed, and if not, alias all of the missing types to Any. Not the best solution, but at least you don't lose the annotations.

[mrava87]

Oh, good idea :) I didn't think about that, but it make sense

[mrava87]

@cako I created a PR, have a look if what I did is in line with your suggestion :)