ENH: Add time utilities to ossim plugins

- Add `ModifiedJulianDate` and `Duration` classes
- Fast conversion function from `"%Y-%m-%dT%H:%M:%S.subsec"` to modified julian date.
  Result is 6 times faster than `ossimLocalTm::setIso8601()`
- Benchmark for new `ossim::toModifiedJulianDate()`

Note, the microbenchmark requires google.benchmark
(http://github.com/google/benchmark). It will be ignored (not compiled) if this
library cannot be found.

To use it, install it somewhere, and run cmake with for instance

```
GBenchmark_DIR=~/local/benchmark/ cmake . -DFIND_COTS_DEBUG=true
 cmake . -DGBenchmark_DIR=~/local/benchmark/
```

(in the case google.benchmark has been installed into `$HOME/local/benchmark`).