consider: remove isotopia dependence

[sebkopf]

consider removing the direct link to isotopia because it can be a potential source of errors and makes installation less straightforward, and is not very essential to the core mission of just reading isotope data (of course once it's being reacluated this could be quite important!).

either way, if doing this, can just add in the intros a few examples how to convert the read data easily to isotopia objects for data conversion