Update data-frame loader

[dtm2451]

This PR:

• adds a 'hole_value' textfield to the data-frame loader config
  • Relatedly, turns off pandas' default na filtration when the data_frame is loaded in to not create additional, hidden to the user, 'hole_value' ignorance. Importantly, this means that this loader can functionally be used to fill attributes as "NA" / "N/A" / "null" if wanted by the user. All of these examples are treated as missing data by pandas.read_table in the default behavior that's turned off here.
• Adjusts the labeling of the 'blank_table' switch

'hole_value' UI:

Previous 'blank_table' look:

non-table model table model

NEW 'blank_table' look:

non-table model table model

[amadeovezz]

Do you have an empty loader definition for the population model? This seems like the catch for a barely initialized script:

Moving this to another thread.

Nope, this is my config. Any ideas?