Refactor function locate via helper class ParticlesLocator

[ivanovmg]

Refactoring

Previously the function trackpy.feature.locate contained a lot of procedures, which made it difficult to comprehend.

The helper class ParticlesLocator was extracted to improve code readability.

The class ParticlesLocator allows the following:

• Validation of input parameters using property setters.
• Readable methods, extracted from commented regions.
• Public method locate, which contains a readable sequence of steps to be carried out before returning the coordinates.