Describe the bug
Strings of Value Representation DS are restricted to a maximum length of 16 bytes according to Part 5 Section 6.2, but pydicom.valuerep.DS may represent numbers with more than 16 bytes.

Expected behavior
pydicom.valuerep.DS should create a string of maximum length 16, when passed a fixed point number with many decimals.

Steps To Reproduce

len(str(pydicom.valuerep.DS(3.14159265358979323846264338327950288419716939937510582097)).encode('utf-8'))
len(str(pydicom.valuerep.DS("3.14159265358979323846264338327950288419716939937510582097")).encode('utf-8'))

returns 17 and 58, respectively, instead of 16.

Your environment

module       | version
------       | -------
platform     | macOS-10.15.6-x86_64-i386-64bit
Python       | 3.8.6 (default, Oct  8 2020, 14:06:32)  [Clang 12.0.0 (clang-1200.0.32.2)]
pydicom      | 2.0.0
gdcm         | _module not found_
jpeg_ls      | _module not found_
numpy        | 1.19.4
PIL          | 8.0.1