Implement currency data update workflow

[Wegatriespython]

This replaces the stubbed iam_units.update.currency() with a working generator and expands currency support from one hardcoded entry (EXC, EUR, 2005) to all five OECD Table 4 methods (EXC, EXCE, PPPGDP, PPPPRC, PPPP41) across five periods (2005, 2010, 2015, 2020, 2024) for EUR. This partly addresses #25.

The original stub in update.py specified three requirements for an implementation:

• Use sdmx1 to query either the World Bank or OECD SDMX API.
• Confirm the different data sources give the same results; if not, expose source selection. Cross-source validation against World Bank was tested manually for two data points (EXC and PPPGDP matched for DEU 2005), but the WDI SDMX endpoint was unreliable (403/404, timeouts), so WB is not wired in as a source. This can be revisited in a follow-up.
• Write to a simple text format readable without dependencies, in iam_units/data/currency/, one file per (method, period).

The inline NB comments in currency.py noted two extensions: loading from file instead of embedded constants, and extending to cover other currencies. This PR addresses the first; EUR remains the only non-USD currency.

The current PR:

• Implements currency() in the update submodule using sdmx1 to query OECD Table 4 (DSD_NAMAIN10@DF_TABLE4), following the pattern established for emissions data.
• Writes plain-text data files to iam_units/data/currency/, committed as package data and loaded at runtime without any dependency on sdmx1.
• Guards configure_currency() against silent wrong answers when called twice with different methods for the same currency/period pair — now raises ValueError. Same-method calls are idempotent.
• Documents the update workflow in DEVELOPING.rst and the expanded capabilities in the README.

EUR rows are derived from the DEU series in Table 4. For exchange rates this is exact; for PPP methods the values are Germany-specific. This follows the approach in #60 which switched EUR deflators to Eurozone-wide values, but PPP is inherently country-level — the choice is documented in code and DEVELOPING.rst.

[khaeru]

From the description this is great, thanks for the contribution!

One knee-jerk question prior to a real review: the 25 added data files each have 1 data point in them. Did you consider / would it be worse/better to follow an approach similar to the generation of emissions.py? IOW, generate a Python source file containing a data structure with all relevant data. This could be more compact.

[byersiiasa]

wow @Wegatriespython didn't expect you to address this within 2 hours. Thank you so much! ;)