Add content sniffing to `utils.files.is_dicom`

[jond01]

Inspired by contrib-pydicom:

https://github.com/pydicom/contrib-pydicom/blob/cbd27f1ac523863aeba820edf796bf040d63993f/input-output/dicom_model/dicom_dir.py#L43-L50

Looks like the following should complement medio's is_dicom function:

def is_dcm(filename):
    with open(filename, 'rb') as f:
        x = f.read(132)
    return x[128:] == b"DICM"

It is particularly useful since many times dicom files lack any extension.
I checked the above is_dcm on some data and it looks reliable.

With that, we can change the default dicom reader backend to pydicom (single and series).

---

Edit:
Even better, use pydicom's built-in is_dicom function (equivalent to the above is_dcm function).