overrride global info when creating new file, not replace

[Teque5]

To create the absolute minimum valid SigMF file we may do something like this:

data = np.zeros(10, dtype=np.complex64)
data.tofile('some.sigmf-data')
meta = SigMFFile(data_file='some.sigmf-data', global_info={'core:sample_rate':1000, 'core:datatype':'cf32_le'})

As is this overrides the default global_info and yeilds:

>>> meta
SigMFFile({
    "global": {
        "core:datatype": "cf32_le",
        "core:sample_rate": 1000,
        "core:sha512": "b6b4c190ccd537f7b879658fb3ab39a81347e1ecb68246dab95648560587b8437ecb02b7825f46a9141059ae61887ec7b011c1314cbcc5741c9fae953eaa02ec"
    },
    "captures": [],
    "annotations": []
})

but it should contain the default specification version & num_channels like:

>>> meta
SigMFFile({
    "global": {
        "core:datatype": "cf32_le",
        "core:num_channels": 1,
        "core:sample_rate": 1000,
        "core:sha512": "b6b4c190ccd537f7b879658fb3ab39a81347e1ecb68246dab95648560587b8437ecb02b7825f46a9141059ae61887ec7b011c1314cbcc5741c9fae953eaa02ec",
        "core:version": "1.2.0"
    },
    "captures": [],
    "annotations": []
})

I should have done this while fixing #67.