distinguish missing from excess timesteps in timesteps validation

[emileten]

@dgergel this is solving #139. We're (1) raising an exception if the time span is too short, with a slightly lower expectation for 'future' 'cmip6' (2) warning the user if the time span is too long.

@brews I don't think there is a test to write, but let me know if I am wrong.

Close #139

[brews]

Thanks for this @emileten!

I can live with this if this fits the behavior and level of testing that @dgergel wants for validation. I'm not sure we need to check "historical" like we've done for "future" because we explicitly crop historical at 1950 -- but again, go with her feel on this.

@emileten, please also add an additional commit noting your change in HISTORY.rst. The easiest way is to copy the PR title, and note your PR number and GitHub name. We want to be sure you're credited for your contributions.

[dgergel]

@emileten looks great - thanks for fixing this! Only one small requested change in the assertion statements

[emileten]

Thanks both. @dgergel @brews could one of you give me write access so that I merge this branch ? Thanks !

[dgergel]

Thanks @emileten! And checking on permissions for you. Merging in the meantime.