Support of missing values for generalized Procrustes analysis

[FanwangM]

When performing generalized Procrustes analysis, it is very likely to have missing values in the configuration matrices. Casper J. Albers and John C. Gower proposed a strategy to deal with such a situation, _Albers, C. J., & Gower, J. C. (2010). A general approach to handling missing values in Procrustes analysis. Advances in Data Analysis and Classification, 4(4), 223-237.. A very nice feature of their algorithm is they can also deal with centring and/or standardisation.

This issue relates to #76.

[aviralgarg05]

I would like to solve this issue

[FanwangM]

Thanks! Please feel free to let me know if you have any questions.
When making pull requests, can you please ensure add the tests as well? We follow the practice of unit test to ensure testing coverage.