fix: mark InSituVariable and MeasuredVariable as abstract (#19)

* mark InSituVariable and MeasuredVariable as abstract

These are intermediate hierarchy classes, not meant to be instantiated
directly. Marking them abstract removes them from the JSON Schema anyOf
discriminator, preventing Claude and other tools from using them as
schema_class values when generating metadata.

Fixes submarine-mrv/oae-data-commons#92

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* improve variable class and enum descriptions for LLM clarity

Class descriptions now explain when to use each concrete variable class
with examples and the expected variable_type/genesis/sampling values.
VariableType enum descriptions now reference which schema classes to use.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* clarify that QC flag columns belong on the parent variable, not as separate NonMeasuredVariables

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* update descriptions

* update non_measured pv description

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: clevinson