Micaps站点数据读取出错，一些包含星号**的数据无法正确读取，建议改进

[jiangleads]

为了读取micaps diamond 1 格式的数据，参考nmc_met_io-master/examples/retrieve_micaps_model.ipynb的相关语句，我使用如下语句读取，但是在遇到错误，不能读取星号数据，建议改进：

>>> from nmc_met_io.read_micaps import read_micaps_1
>>> data = read_micaps_1("/mnt/d/datafile/surface/plot/20020402.000")
Traceback (most recent call last):
  File "pandas/_libs/lib.pyx", line 1926, in pandas._libs.lib.maybe_convert_numeric
ValueError: Unable to parse string "****"

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/nmc_met_io-0.1.0-py3.6.egg/nmc_met_io/read_micaps.py", line 95, in read_micaps_1
  File "/usr/local/lib/python3.6/dist-packages/pandas/core/tools/numeric.py", line 150, in to_numeric
    values, set(), coerce_numeric=coerce_numeric
  File "pandas/_libs/lib.pyx", line 1963, in pandas._libs.lib.maybe_convert_numeric
ValueError: Unable to parse string "****" at position 1021

[NMC-DAVE]

已经增加对非数字型字符串的处理，目前"****"将会处理成NAN值，可以见例子 https://github.com/nmcdev/nmc_met_io/blob/master/examples/retreive_micaps_files.ipynb