Numeric holding ids in from/to don't get converted to characters

[CarlijnB]

bug report from Emily Hotchkiss, who tried the workshop version of the app.

With a movement datafile with numeric holding identifiers (to/from fields), the app errors-out at movedata2networkDynamic:

Unhandled promise error: Assertion on 'movement_data[[1]]' failed: Must be of type 'character', not 'double'.                                             
NULL
Warning: Error in movenetapp::movedata2networkDynamic: Assertion on 'movement_data[[1]]' failed: Must be of type 'character', not 'double'.