Add single-sided buffer functionality from geos

[stev00]

I noticed that the ST_Buffer function does not implement the single-sided option available in geos (since CAPIv1 I think looking at git blame).

I have added a patch to address this. It is not yet comprehensive as, being a new contributor, I would like some feedback on what the function signature should be etc.

At the moment, it is implemented as a text string in buffer_style_parameters as this seemed like an appropriate option, but very happy to be corrected on this. It is a slightly messy implementation, as the parameter to the geos function is an integer, so I have kept it as that, although it could easily be a text 't' or 'f' or 'true' or 'false'. I also noted that the other parameters didn't raise any exceptions if input wasn't as expected, so have retained this behaviour.

I have done some limited testing, and run regression tests with no failures. I have not yet written additional tests and documentation, but will do that if and when the broad approach gets some sort of thumbs-up.

[stev00]

Code only patch

[stev00]

Just to add, the reason it has been implemented in the buffer_style_parameters parameter, is to avoid changing the signature of the function. People may feel it's more appropriate to have a separate function signature, but there are already 6, and I thought more would confuse further.

[pramsey]

Sorry this took so long to review.

[dbaston]

Committed to trunk at r16380, modified from ​https://github.com/postgis/postgis/pull/204

[robe]

In 16431:

Add ST_Buffer side=arg enhancement not to the docs.
References #3989