improved documentation

BaseStorage.java
    stores an amount without error as simple double / unit combination
        not needed and causes problems
        added protected accessors
ConfigurableStorage.java
    removed storeError flag
    added store method
    updated LimitedTestStorage and test
AbstractLimitedStoragePipeline.java
    delayed storages are hold back when removal is rejected by sum storage
    added store method
    added toString method in properties proxy displaying simple class name
added AmountIsCloseTo.java Matcher class for tests