Add ObserveHasAnyDatoms

[erri120]

Resolves #91

[halgari]

I have a few problems with this PR:

1. it performs a new query on every database update, so if the datom selection is large it will need to read in a ton of datoms every time
2. it's a count on an IndexSegment. So we're reading in the entire segment just to check the size, then terminating with "if the size > 0". So we either should create a iterator and terminate after the first found entry or do something else

We should either use a iterator and terminate early or run this against the AddedDatoms property