Fix inconsistencies

[Querz]

This pull request fixes some issues regarding inconsistency.

• All Tags now implement Comparable on their own to assure type safety
• ListTag and CompoundTag now always clear their values before deserialising data into them
• Tag#toString() and Tag#toTagString() are now final to force child classes to use the maxDepth parameter
• Passing null to #compareTo now throws a NullPointerException
• Calculation of the maximum depth is inverted to allow a custom maximum depth
• Updated the javadoc for proper use of the maximum depth
• Negative ListTag and StructTag sizes are corrected to 0
• Removed Tag#getEmptyValue()
• Added unit tests for all changes

[coveralls]

Coverage increased (+0.003%) to 98.955% when pulling 66ce5295931484b0d55722ca1c81b80a5131ad80 on inconsistencies-fix into 74e37d4986a27b66113abd2c59a9d78e6c9a4900 on master.

[Marcono1234]

These points do not apply anymore

• ListTag and CompoundTag should throw an exception when comparing them with incompatible types
• ListTag and CompoundTag should always clear their values before deserialising data into them
• Add ListTag#sort() to make use of the implementation of Comparable in each tag

And you could change NumberTag and ArrayTag to be type-safe for compareTo at the cost of a slightly more complicated signature:

public abstract class NumberTag<T extends Number & Comparable<T>, ThisTagType extends NumberTag<T, ?>> extends Tag<T> implements Comparable<ThisTagType> {

And then have subclasses add their own type as ThisTagType:

public class ByteTag extends NumberTag<Byte, ByteTag>

Though this makes using NumberTag more unpleasant. Maybe it would be best to just only have the subclasses (e.g. ByteTag) implement Comparable and not NumberTag.

[Marcono1234]

The changes look good. Thank you very much!