Custom solute location and disablable unpinning mechanisms

[arn-all]

• Allow bypassing random Cottrell atmosphere generation by providing a JSON formatted file 37aa5be394f19f89928d346f153fd2af444a9168
• Add post processing script to extract dislocation velocity & compare to analytical expression with no solute 52be8697f7ab7b321395a5eb8a0d111bd2397725
• Allow disabling pinning mechanisms ( #4 ) except annihilation
• Moved some hard-coded parameters to the config file #2
• A bit of cleaning

Note : now ships with the module https://github.com/nlohmann/json which is under MIT Licence.