Also
- simplify and add tests
- remove large memory leak (persistant store of all non-project layers)
- remove broken support for direct loading postgres/virtual layers
by string (Python version was very broken and would never match
a postgres/virtual layer)
Replaces the existing 'Basic Stats for Numeric Fields' and
'Basic Stats for String Fields' algorithms and adds support
for date/time/datetime fields.
Having a single unified algorithm allows more flexible models
where a field type may not be known in advance.
Deprecate existing basic stats algorithms
