new option to ignore/overwrite .heudiconv/ conversion information/assignment

[yarikoptic]

e.g. so we could re-convert data within the same dataset without using original paths.

may be smth like --ignore-heudiconv

[yarikoptic]

related use case just came up -- when user wants to reconvert only a subset of files, specifies them in --files but then heudiconv picks up all files for that subject/session to be reconverted. Probably worth getting back to #134 and see what we should do about that.