Spark 3.2 (swoop-inc#9)

* Bump spark version to 3.2.0
* Bump alchemy version for dependency change
* Code updates for changes to Spark APIs

Spark 3.1.2 support (swoop-inc#22)

* Spark 3.1.2 support
* Bump version for dependency change

Upgrades to Spark 3.0.0 and Scala 2.12.11

Add Aggregate Knowledge HLL implementation

along with the ability to choose which implementation is used.

* Choose HLL implementation by:
   - SparkSession config OR
   - explicit argument to functions
* Postgres interoperability tests for Aggregate Knowledge HLLs
   - launches Postgres with the HLL extension installed, in a Docker container
* `hll_convert` for converting sketches from StreamLib to Aggregate Knowledge

Also:
* Add code style & apply
* Move release process info to DEVELOPMENT file

Bump version to 0.3.0 for release

Adds HyperLogLog++ functions for Spark

Includes native function registration, and a hashing strategy that handles all Spark datatypes

Implemented new project structure and release process