A function which drops all non numerical columns

[Gerhardsa0]

Is your feature request related to a problem? Please describe

Sometimes we want to drop all non numerical columns, so we can easily plot an heatmap.
A function which drops all non numerical rows and returns a new table could work well here.

Desired solution

Table.drop_non_numerical_columns()-->Table

[lars-reimann]

The transformation can already be achieved by writing Table.from_columns(table.list_columns_with_non_numerical_values()). For the plotting methods we should maybe offer a parameter to decide whether to raise an exception when invalid columns are passed or whether to ignore them.

[lars-reimann]

This is done now:

• correlation_heatmap now can be called on a Table with some non-numerical columns (feat: allow calling correlation_heatmap with non-numerical columns #92).
• boxplot and histogram are called directly on a Column, so raising an exception makes sense.
• lineplot and scatterplot are called on a Table but columns are specified explicitly by their name. Raising an exception again makes sense.