handle non-identical meta

[jonathancallahan]

Saw this when combining AirNow data from 2016-2020:

 Error in MazamaTimeSeries::mts_combine(...) : 
  The following ids have non-identical metadata: 950c48de00764e18_530730015_01, 71fee2cf0d6f2db8_360050110_01, 7a53d08050826781_090039999_01, 630f798bdf6f4980_160871004_01, 0bf8f8ed05f7ecf4_069999917_01, 7213214b323b6339_420050001_01, a66b77a32deac6d2_420430401_01, ce2244ca974f403c_420770004_01, 6eca73053c6cb209_421174000_01, 7e2651ce24352ceb_120110033_01, f6b4431a3529dad4_311090022_01, 009a93fd79b28845_420710007_01, 3e7e1e0bb4c7b88c_360010005_01, 3858898d0bfd9a5c_360810120_01, b4cac1a3c18fcbf0_360850111_01, 541ff3e07383d7c6_420750100_01, ecdafb2086781ebb_000090133_01, 7c74533d6c40b51f_420070014_01, c9c367872ac8f47a_420210011_01, cf87a5d2d12ab6ce_420910013_01, 8c92148d144fc39d_421255001_01, b9f49295427da8b4_360652001_01, 9ee303bee0feec3b_800020014_01, 590324ebbb1c8896_530630021_01, dfef878815787e3e_120110035_01, d51f564b870a8b5a_360710002_01, 3bfe7da06af22343_201950001_01, 8815e407741361a7_420290100_01, 63ff78a5ce557030_420710012_01, d9619e5d0eaf6155_420410101_01, 17a59f433eff57 

We should have a flag to allow later-is-better merging of meta