Improve CDP4Dal.Operations.ThingTransaction initialization and update some documentation

[IevhenIkonnykov]

Prerequisites

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Description

In the second constructor move this.TransactionContext = parentTransaction.TransactionContext to the if statement where parentTransaction is checked for null.
Fix word "ass" to "add" in docs for AddCloneToContainer and UpdateContainer methods.

Steps to Reproduce

See the source code.

System Configuration

Does not matter.

• CDP4 version:
  • CDP4Common:
  • CDP4JsonSerializer:
  • CDP4Dal:
  • CDP4JsonFileDal:
  • CDP4ServicesDal:
  • CDP4WspDal:
  • Other:
• Environment (Operating system, version and so on):
• .NET Framework version:
• Additional information:

[IevhenIkonnykov]

Merge method contains the following piece of code:

// check if the key in a sub-transaction correspond to a value in the current one
var existingKeyValue = this.updatedThing.SingleOrDefault(x => x.Value == keyValuePair.Key);
if (existingKeyValue.Key != null)
{
       this.updatedThing[existingKeyValue.Key] = keyValuePair.Value;
 }

which looks incorrect, because we might get NullPointer if existingKeyValue is null. There should be
if (existingKeyValue != null)