readChangeoDb should probably convert all columns to character type

[Jason Vander Heiden]

Have readChangeoDb read all columns as strings and selectively convert fields to numeric (DUPCOUNT, JUNCTION_GAP_LENGTH, etc) or boolean (FUNCTIONAL, etc) to avoid issues when sample ids (for example) may be numeric.

[Jason Vander Heiden]

Let default imports remain, but added explicit conversion of textual fields to character.