Add South Korea fetcher (Fixes #72)

[thiagovmdon]

Related to issue #72

Implements data fetcher for South Korea.

Includes:

• Metadata scraping (and cached data)
• Standardized coordinate transformation (to decimals WGS84)
• Variable support for stage (daily and hourly) and discharge (daily)
• Add South Korea to docs, and local test (it seems to be working)
• Missing tests

[thiagovmdon]

@kratzert , I downloaded and checked many stations visually, and I found some strange patterns sometimes:
It seems that either we have stations with years of 0 flow, or the API returns some 0s where it should be missing... It happened only in some basins, so as an advice, maybe it would be interesting to have a look before using it! I do not believe there are such cases of 0s, but trying to be the devils advocate, perhaps they have some very specific hydrology taking place in Korea? Anyway, I found it worth noting for you!

[kratzert]

Do you have any ids for which this happened? Would maybe facilitate the research

[thiagovmdon]

Ow, silly me, for sure it is needed the ids. Sorry Freddy. Here it goes some examples (for streamflow): 1018680 (from 2010 onwards), 1001605 (2013/2014), 1001607 (2015 onwards). For stage I did not get any issue.