Bump version to 1.9.1

This new release updates three main crossover operators to perform also with chromosomes with different lengths.

New updated selection operators.

Added new selection operator (MetaSelector).

v. 1.8.1

Connects this version with Zenodo.

New multi-modal support feature.

Added new selection operators to support multi-modal optimization problems.

New updates and bug fixes.

v.1.7.5

Rerun all examples.

Version (pre-release) at acceptance of the paper

This version includes bug fixes and optimizations. Most notably;

1. the crossover operators use the clone method to avoid problems
   with referencing the same genes in different members of the population.

2. The deepcopy methods in Gene and Chromosome have been customized
   to improve performance.

3. The Tournament Selector has been optimized to avoid loops.

v.1.6.8

Several new changes:

fix typos and bugs,
refactoring methods for better performance,
rebuilt documentation

also this version includes new features such as:
i) a new termination condition
ii) adaptive crossover and mutation option.

v.1.6.5

Latest performance updates and rebuild documentation (including pdf).

v1.6.0

Several major performance improvements over the previous version.

v1.5.0

Some updates, regarding v1.0.0 are: i) the incorporation of several new genetic operators, ii) improvements in the crossover operators to avoid cloning the chromosomes (improves speed), iii) addition of several new examples.