Support all exotic number formats excel might export

[jheeffer]

Find a library that can parse numeric strings while taking into account weird thousand and decimal separators (e.g. 100 395,395). Use it to test if a concept is a measure or string.

Helena (Nordenstedt) had this error when her excel exported with space as thousand separator.

[jheeffer]

It might not be possible to do this umambiguously. E.g. in 100.000 the . can be a thousand and decimal separator. We probably want two parameters for the two separators which default to "auto", meaning we try our best to guess ; ).