* implement procedure of MSDO/MDSO in p29

* undocumented functions include findParsimonious, overlap, which.commonConfiguration.

* plot method for QCA object with aids from Vennerable packages.

* Can use gtools to improve speed of solvePIChart(), but not use it until necessary. The other option is to use commbinations in omegathat or use Rcpp to use C++ code instead of R code.

* handle necessary conditions.

* examine the logical relation between the derived variable and original variables (for TQCA).

* consistency for ~Y in fs_truthTable()

* bool calc

* how to handle "don't care" condition- 1) Get the realised value for such condition before construct the PIChart; 2) simply treat the don't care value whatever is easy for the PIChart to get a TRURE?
