Add method to add columns for the Python API `Engine`

[Michael T Schmidt]

There is no way to add a column once an engine is created. An empty Engine would be instantiated for the required use case, and custom columns would be added to it.

General idea (not a specification):

from lace import Engine, ColumnType, Normal

eng = Engine()
eng.add_column("column_name", ColumnType.Continuous(Normal.standard()))

eng.add_row({
    "column_name": 1.234,
})

[Baxter Eaves]

This is addressed in #95