Better identification of configured layers without (valid) data

[Joern Ungermann]

The server is configured with certain layers. When creating the capability document, the server goes through the data files and assembles init and valid times for each layer.

In case no valid files are found, the layer should not be offered to the client, as this will provoke error messages on the client side.

Instead a noticeable error message should be printed to the server log file.

[Reimar Bauer]

Check in WMS that a time-using layer has actually times

This adds a validation check to WMS server to offer only layers with valid configuration. Invalid layers will be flagged in the log as ERROR.

Resolve Issue #101

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