Mixing rule of liquid density

Changed mixing rule to wt% instead of mol%
Merge pull request #25 from waqqasdadabhoy/master

Don't close without selecting Components/Property Package.
Merge pull request #26 from waqqasdadabhoy/master

Select Components and Property Packages by double-click.
Update Optimization.vb

Fix for XML persistence.
Update SensitivityAnalysis.vb

Fix for XML persistence.
Update SteamTablesPropertyPackage.vb

Fix for bug #13 => https://sourceforge.net/apps/mantisbt/dwsim/view.php?id=13
Small GUI adjustments for improved Mono compatibility.
Update FormMain.vb

Updated XML loading code to store exceptions and show them later after completing loading instead of breaking the process in the middle.
Update Optimization.vb

Force to not save the results in the XML file.
Update SensitivityAnalysis.vb

Force to not save the results in the XML file.
Update FormSensAnalysis.Designer.vb

Decreased window size. Will redesign it later.
Update README.txt
Integration of Liquid Heat Capacity regression into Compound Creator

Integration of Liquid Heat Capacity regression into Compound Creator.
No Estimation method available yet. Only regression.
Update BostonFournierInsideOut3P.vb

Fixed resulting vapor composition in three-phase PT-Flash.
Update Vessel.vb

Changed flash calculation type to increase reliability for single-compound simulations.
Update Vessel.vb

Fixed equilibrium calculation for a single compound mixture.
Update FlowsheetSolver.vb

Fixed flash calculation for single compound ignoring the defined specification.
Update Excel.vb

Added functions to return interaction parameters stored in DWSIM for a given binary / model.
Update PropertyPackage.vb

Fixed chemsep database search for excel/cape-open mode.
IO flash tweaks.