Add functions to list variables available in a .sql file (#17)

[Tokarzewski]

Summary

Implements #17 — a way to discover what variables a .sql file contains, grouped by table (reporting frequency). Adds the three methods suggested in the issue:

fromdb_eplusout_readerimportget_tables, get_variables, get_all_variablesget_tables(path) # -> ['timestep', 'hourly', 'daily', 'monthly', 'runperiod']get_variables(path, H) # -> [Variable(key='Environment', type='Site Opaque Sky Cover', units=''), ...]get_all_variables(path) # -> OrderedDict {frequency: [Variable, ...]}

• get_tables returns the distinct frequencies as {TS, H, D, M, A, RP} constants, de-duplicated (Zone Timestep / HVAC System Timestep both map to TS) and ordered most→least granular.
• get_variables returns the sorted Variable list for one frequency.
• get_all_variables composes the two into a {frequency: [Variable, ...]} overview.

All three are exported from the package top level. A small connect() helper centralizes the file-existence check, now also reused by get_results_from_sql.

Tests

Added TestListVariables (runs across all versioned SQL fixtures): table discovery + de-dup + ordering, per-frequency variable listing, a non-meter round-trip through get_results, get_all_variables consistency, and IOError on a missing file. Full suite: 130 passed; ruff clean.

Note

Listed meter variables carry key='' (NULL KeyValue) and currently aren't retrievable by exact key match — that's the limitation tracked in #12 (fixed separately in #21). Listing them here is correct; only exact-key retrieval is affected.

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