Plumbing for pipelines (#453)

* re-enter pipeline

The pipeline interface is now internally based on the pj_obs_api, which
simplifies the implementation significantly.

This is the first mock up - it compiles fine, but is currently untested

* pipeline code cleaned up

The pipeline code is now based on the PJ_OBS api (although you can still
invoke a pipeline through pj_fwd/pj_inv and their 3D brethren).

This has made it possible to eliminate scores of funky casts and
convoluted workarounds. The code is now way more straightforward and
mostly conforming with common C idioms..

Also, the proj.h / obs_api interface to the logging system has been
streamlined through the introduction of the pj_log_error, pj_log_debug,
and pj_log_trace functions.

* Geodesics + minor changes

First proj.h style interface to Charles Karney's geodesics code:
pj_lp_dist.
Also, renamed pj_apply -> pj_trans

* Extended Ellipsoidal Parameters

Second eccentricity, second and third flattening etc.

* Rename pj_debug_set -> pj_log_level

... and add self test code for PJ_pipeline

* Clean up missing pj_apply->pj_trans

* Clean up missing pj_obs_dist_2d rename

* pj_strerrno bug fixed. Some doc/comments added

(In response to a review by @kbevers)

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