Small update to allow for batch insert/update operations on entities with computed properties

[jlikens]

• Added IsComputed to PropertyMapping
• Set PropertyMapping.IsComputed for scalar properties based on prop.Column.IsStoreGenerated
• Changed InsertAll and UpdateAll to skip computed columns

Was unable to add proper tests as there does not appear to be a database creation script in the solution and I didn't feel like reverse engineering one

[oryol]

Can it be fixed without manual patching and rebuilding library? NuGet package doesn't work well with computed columns