Typing of sentStats::sv should ideally be a wordStats instead of a basicStats

See:

https://github.com/UUDigitalHumanitieslab/tscan/pull/61#discussion_r990038148

The problem is that `struct basicStats` (which is the base for `wordStats`) contains the definition:

https://github.com/UUDigitalHumanitieslab/tscan/blob/bec422ccb74a3c88ef329dfbc0b925021d0bf8fa/include/tscan/stats.h#L107

This makes it harder to introduce generic type because this would introduce a circular type dependency e.g.:

```cxx
typename T
struct basicStats {
...
std::vector sv;
}

struct wordStats: public basicStats> {
...
}
```

Start with the `struct basicStats` definition in `include/tscan/stats.h` and read the linked discussion in pull request #61. Trace how `sentStats::sv` and `wordStats` are related, then define what type relationship would avoid the proposed circular dependency; done means the typing design is agreed and implemented without that dependency.

Written by the indexing model from the issue text.