Fixes #2482

And also add proj_context_errno_string()

Revise gie 'expect failure errno XXXX' strings

Add +proj=topocentric geocentric->topocentric conversion (fixes #500)

Gone are pj_malloc, pj_calloc, pj_dalloc and pj_dealloc. Their primary
function as API memory functions in proj_api.h is no longer there and
the other use as a workaround for old errno problems is no longer valid
either.

Replaced with malloc and free across the codebase.

Add proj_degree_input() and proj_degree_output()

Equivalent to proj_angular_input() and proj_angular_output() but
checking for degree units instead. proj_create_crs_to_crs() rarely, if
ever, returns pipelines that has radians as input or output so using
proj_angular_*() is not a useful check for io units of pipelines.

These two new functions should make life a bit easier for users that
generally store there angular coordinates in radians.

Closes #2027

Saves 2 sincos() and 1 atan2() calls.

With the following bench
```
 #include "proj.h"
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

int main(int argc, char* argv[])
{
    if( argc != 2 ) {
        fprintf(stderr, "Usage: bench_inv_cart fwd/inv\n");
        exit(1);
    }
    PJ* p = proj_create(0, "+proj=cart");
    const int dir = strcmp(argv[1], "inv") == 0 ? PJ_INV : PJ_FWD;
    PJ_COORD coord;
    if( dir == PJ_FWD )
    {
        coord.xyz.x = 3.14159/2;
        coord.xyz.y = 3.14159/2;
        coord.xyz.z = 100;
    }
    else
    {
        coord.xyz.x = 3e6;
        coord.xyz.y = 3e6;
        coord.xyz.z = 3e6;
    }
    for(int i = 0; i < 10* 1024 * 1024; i++ )
    {
        proj_trans(p, dir, coord);
    }

    proj_destroy(p);
    return 0;
}
```

On Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
Time before: 2.37s
Time after:  1.57s