Support for seasonally adjusted construction starts data (table 5.3.3)

[bdbmax]

We are trying to retrieve the seasonally adjusted annual rate of construction starts, and found it's not available through the package API. It's available on the portal down to CMA-level, table 5.3.3.

I'm following along the get_cmhc and just swapped the TableId before the POST call of a another minimal request, and correctly extracted Starts (SAAR). The correct csv gets read, but I understand the rest function isn't written to deal with its structure.

I think this case is about:

1. expanding the list_cmhc_tables generation to deal with this additional table. Were those tables handwritten manually?
2. review how get_cmhc could handle this table format.

Does this make sense to you? Let me know, I could try a PR on this.

[mountainMath]

Yeah, tables are handwritten manually. I have not been able to get a comprehensive list. It's all a bit hacky, I added some code to allow for querying the seasonally adjusted starts in #28. It's only available at the "Centre" (CMA) and higher geographic levels. For calling this table the Dimension argument should be set to NA or NULL. Sample code:

get_cmhc("Scss", "Starts (SAAR)", NULL, "Historical Time Periods", geo_uid="59933")

[mountainMath]

Thanks for offering to help out, given the hacky nature of this project I figured it's easier if I go in and do the adjustments.

One afterthought, I will ask CMHC which method they use for seasonal adjustment next time I talk to them, it might be useful to just implement seasonal adjustment in the package. It would have the advantage that it could run on all series, still conform to the CMHC series for when they provide it, and allow for fine-tuning if needed.

[bdbmax]

Okay, that works. Thank you for #28 ! And it's a great idea to ask about their methods. I agree with the advantages!