new ToString for DateTimeInterval

Signed-off-by: Emmanuel Mathot <emmanuel.mahot@gmail.com>
Merge branch 'master' of github.com:Terradue/DotNetOgcOwsContext
1.0.0

Signed-off-by: Emmanuel Mathot <emmanuel.mahot@gmail.com>
Merge branch 'master' of github.com:Terradue/DotNetOgcOwsContext