Implement TextGrid#toString for easier debugging, etc.

[psibre]

Expected behavior

TextGrid#toString should be implemented for human inspection.

Actual behavior (with the stacktrace!)

This is not really helpful:

Gradle suite > Gradle test > org.m2ci.msp.jtgt.io.TextGridSerializerTest.testEquals FAILED
    java.lang.AssertionError: expected [org.m2ci.msp.jtgt.TextGrid@3538da99] but found [org.m2ci.msp.jtgt.TextGrid@41086e4c]
        at org.testng.Assert.fail(Assert.java:94)
        at org.testng.Assert.failNotEquals(Assert.java:494)
        at org.testng.Assert.assertEquals(Assert.java:123)
        at org.testng.Assert.assertEquals(Assert.java:165)
        at org.m2ci.msp.jtgt.io.TextGridSerializerTest.testEquals(TextGridSerializerTest.java:69)

Steps to reproduce the behavior

Checkout 122a8e4 and run ./gradlew check.

Additional information

Indicate any necessary additional information