Add IEnumerable<double> Samples to ISampleMonitor (#31)

The field `IEnumerable<double> Samples` is already present on
`SampleMonitor`, but it's missing from `ISampleMonitor`, which requires
a (potentially dangerous) cast.

To make the cast unnecessary, add it to the interface.

src/SimSharp/Analysis/IMonitor.cs

@@ -6,6 +6,7 @@
 #endregion
 
 using System;
+using System.Collections.Generic;
 
 namespace SimSharp {
  public interface IMonitor {
@@ -34,6 +35,7 @@ public interface INumericMonitor : IMonitor {
 
  public interface ISampleMonitor : INumericMonitor {
  void Add(double value);
+ IEnumerable<double> Samples { get; }
  }
 
  public interface ITimeSeriesMonitor : INumericMonitor {