Refactor datasets to allow the storage of strings and datetimes

[mkommend]

Currently only double values can be stored in the Dataset. Although these are the only values that can be used for modeling, string and DateTime values are useful for information purpose.

[mkommend]

r6740:

• Corrected TableFileParser to handle empty rows correctly.
• Refactored DataSet to store values in List<List> instead of a two-dimensional array.
• Enable importing and storing string and datetime values.
• Changed data access methods in dataset and adapted all concerning classes.
• Changed interpreter to store the variable values for all rows during the compilation step.

[mkommend]

The SymbolicDataAnalysisExpressionTreeILEmittingInterpreter must be adapted to work with r6740 correctly.

[gkronber]

r6741: adapted IL emitting interpreter to work with previous (r6740) changes of the dataset.

[mkommend]

r6749: Added Storable attribute to dataset values.

[gkronber]

r6754: fixed a problem with empty trainingindizes when using cross-validation

[gkronber]

r6769: fixed a bug in the interpretation of lagged variables introduced with r6740.

[gkronber]

Importing data does not work reliably. In the TableFileParser lines 100 - 122 first the type of a column is determined heuristically and then the parsed values are filled into the columns.

First of all the statement in line 100:

var columnType = types.GroupBy(v => v).OrderBy(v => v).Last().Key;

throws an exception because it is not possible to use an enumerable as key. Possibly OrderBy(v=>v.Count()) was meant?

The next problem is that first the type of column is determined by the majority of parsed elements for that column, however, later the elements are simply pushed into the columns without checking if the type is compatible. This throws an exception again.

[gkronber]

r6776 fixed a bug in parsing datetime values and improved code for filling dataset columns

[mkommend]

Reviewed and tested r6741, r6754, r6769 and r6776.