Add support for unnamed ``implicit'' populations and generators

[fsaad]

As a midway point between (i) having a table, population, and generator be identified by a single name to reduce the number of names that an end user needs to know about (in the most common case), and (ii) keeping the full generality of multi-population tables and multi-generator populations that is implemented throughout bayeslite (see #588 for full details), I propose implementing ``implicit'' populations and generators, which are created in the following way:

CREATE TABLE satellites FROM 'satellites.csv';
CREATE POPULATION FOR satellites (GUESS STATTYPES OF (*));
CREATE GENERATOR FOR satellites USING loom (<modeling>);

In this workflow, the user only specified the name ''satellites'' of the base table, and created a population and generator without creating new names. Therefore, the user need not manage multiple names such as ''satellites'', ''satellites_p'', and ''satellites_cc''.

• grammar.y: Add support for unnamed populations/generators.
• schema.py: Add implicit column to the bayesdb_population and bayesdb_generator tables.
• schema.py: Add sqlite triggers enforcing invariants for implicit populations/generators in schema
  • Names must be identical.
  • Table with implicit population can only have one and only one population.
  • Population with implicit generator can have one and only one generator.
• bql.py: Update interpreter to handle unnamed populations/generators, marking them as implicit.
• bql.py: Update interpreter to handle renaming tables/populations with implicit populations/generators, by propagating the renaming downwards (blocked on Implement missing RENAME POPULATION command #598).
• core.py: Add some utitlies for implicit generators/populations, such as checking whether a table/population has an implicit population/generator, respectively.
• test_core.py: Add tests for this whole shebang, including creating, triggers, dropping, duplicates, etc.
• doc/bql.rst: Document using implicit populations/generators in create/rename modeling commands.

[fsaad]

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