readme: Use ellipsis for some doctests

Apparently the MacOS output differs in the last digits of some of the
numbers. Without obfuscating the examples too much, uses ellipses
(`...`) to indicate possibly non-matching digits. Having about 15
significant digits that match should be good enough.

README.md

@@ -135,8 +135,8 @@ and kilograms instead of centimetres and grams (`0` is the default).
 ```python
 >>> from datetime import datetime
 >>> from nrlmsise00 import msise_model
->>> msise_model(datetime(2009, 6, 21, 8, 3, 20), 400, 60, -70, 150, 150, 4, lst=16)
-([666517.690495152, 113880555.97522168, 19982109.255734544, 402276.3585712511, 3557.464994515886, 4.074713532757222e-15, 34753.12399717142, 4095913.2682930017, 26672.73209335869], [1250.5399435607994, 1241.4161300191206])
+>>> msise_model(datetime(2009, 6, 21, 8, 3, 20), 400, 60, -70, 150, 150, 4, lst=16)  # doctest: +ELLIPSIS
+([666517.690495152, 113880555.97522168, 19982109.255734544, 402276.358571251..., 3557.46499451588..., 4.074713532757222e-15, 34753.12399717142, 4095913.2682930017, 26672.73209335869], [1250.5399435607994, 1241.4161300191206])
 
 ```
 
@@ -246,8 +246,8 @@ by importing `nrlmsise00._nrlmsise00`.
 ```python
 >>> from nrlmsise00._nrlmsise00 import gtd7, gtd7d
 >>> # using the standard flags
->>> gtd7(2009, 172, 29000, 400, 60, -70, 16, 150, 150, 4)
-([666517.690495152, 113880555.97522168, 19982109.255734544, 402276.3585712511, 3557.464994515886, 4.074713532757222e-15, 34753.12399717142, 4095913.2682930017, 26672.73209335869], [1250.5399435607994, 1241.4161300191206])
+>>> gtd7(2009, 172, 29000, 400, 60, -70, 16, 150, 150, 4)  # doctest: +ELLIPSIS
+([666517.690495152, 113880555.97522168, 19982109.255734544, 402276.358571251..., 3557.46499451588..., 4.074713532757222e-15, 34753.12399717142, 4095913.2682930017, 26672.73209335869], [1250.5399435607994, 1241.4161300191206])
 
 ```