Faster guessing of CxoTime-specific formats

[taldcroft]

Description

When guessing greta, secs, or date format inputs, this requires that the inputs have the right type (string, number, string, respectively). This shaves off some time that would be spent uselessly guessing, and also fixes a collision for times in secs that look like a valid greta time. [GRETA times do currently accept a float/int input, following the behavior of MATLAB tools].

Testing

• Passes unit tests on MacOS (note: tests cover the case of supplying each of the guessed formats without providing the format arg.