Add MultiValueColumn and both loading and saving logic

[lisad]

A MultiValueColumn needs a bit of configuration

• Delimiters
• Sub-delimiters?
• An idea what type the values between the delimiters should be - cast them?

During load, parse the list into its multiple values:
e.g. df['languages'] = df['languages'].str.split(',')
But also we should strip spaces off of values after split which this doesn't do

During save, rejoin with commas and enclose in double-quotes (only double-quotes work)

Use case: lat,long is a list of values in one column - they should both be validated to be floats

[lisad]

This would be a good one @YuliaS.

To make testing a little easier: In test_reshape.py, there's an example of using a data file 'languages.csv' that contains a multi-value column 'languages'. A test using the same datafile but for a "MultiValueColumn" feature, could be to load the same test file, declare the languages column to be a "MultiValueColumn", and make sure that the value for that column is a list every time a row has that column; also that it's saved out again in a consistent way.