Read solved variables with get_var_primalresult to avoid incorrect column indices after presolve.

[rwe]

This fixes use of incorrect column indices when the presolver removes rows or columns. A serious but silent error when using presolve_cols.

[rwe]

Of my several pull requests to this project (awesome work by the way!), this is the only one that I feel is particularly necessary. When a presolver is run, the matrix's indices can be altered and some even removed, which means that getSolution and getSolutionDict return silently incorrect results.

It's possible to get the stil-existing new column indices back either by name or get_orig_index, get_origcol_name/get_col_name, etc, however those do not provide values for removed columns (for example, when presolve_cols determines its value, saves it, and removes the column for the solver).

According to the documentation:

To retrieve the variable data of all variables, including the presolved variables, API call get_var_primalresult must be used. This is the only call available to get all information.