One source of introducing layers

[layik]

In #38 we have a method to define DeckGL layer properties and use them either in generating layers or indeed in setting their values or assigning values from columns.

For this issue:

• one source of truth where Layer is defined and used consistently throughout
• define current set of layer props

For the latter, we can have:

1. number (numeric)
2. column (value from column), this is turned into a function on the fly
3. colour (array) is used in DeckGL layers

This could do most of what eAtlas needs to do.