Extended tmerc (Poder/Engsager): speed optimizations

fwd: 52% faster on Core-i7@2.6GHz, 40% faster on GCE Xeon@2GHz
inv: 24% faster on Core-i7@2.6GHz, 36% faster on GCE Xeon@2GHz

Those speed-ups are obtained due to elimination of a number of
transcendental functions (atan, sincos, cosh, sinh), through the
use of usual trigonometric/hyperbolic formulas.

The numeric results before/after seem identical at worse up to
14 decimal digits, which is way beyond the apparent accuracy of the
computations

On a point, far from central meridian, where round-tripping is not so great:

Before:
echo "81 1" | src/proj -d 18 +proj=utm +zone=31 | src/proj -d 18 -I +proj=utm +zone=31
80.999994286593448578	0.999987970918421620

After:
$ echo "81 1" | src/proj -d 18 +proj=utm +zone=31 | src/proj -d 18 -I +proj=utm +zone=31
80.999994286593448578	0.999987970918422175

The benchmarking program used is:
```
 #include "proj.h"
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

int main(int argc, char* argv[])
{
    if( argc != 3 ) {
        fprintf(stderr, "Usage: bench quoted_proj_string fwd/inv\n");
        exit(1);
    }
    PJ* p = proj_create(0, argv[1]);
    const int dir = strcmp(argv[2], "inv") == 0 ? PJ_INV : PJ_FWD;
    PJ_COORD coord;
    if( dir == PJ_FWD )
    {
        coord.xy.x = 0.5; // rad
        coord.xy.y = 0.5; // rad
    }
    else
    {
        coord.xy.x = 450000; // m
        coord.xy.y = 5000000; // m
    }
    for(int i = 0; i < 40 * 1000* 1000; i++)
        proj_trans(p, dir, coord);
    proj_destroy(p);
    return 0;
}
```

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