Tables with same dtype names should have the same dtype

Summary

(Summarise the bug encountered concisely.)

Environment

• KM3Pipe version (km3pipe --version):
• Python version (python --version):
• OS: (uname -a)

Additional information

(The following are optional but may be relevant.)

• HDF5 metadata (h5info /path/to/relevant/file.h5)
• ROOT file metadata (JPrintMeta -f /path/to/relevant/file.root):
• Jpp version (only if relevant, JPrint -v)

What is the current bug behavior?

Two tables initialised with dicts containing the very same keys but different order is handled differently in Python 2 and 3. We should always get the same numpy dtype (maybe order by name?)

What is the expected correct behavior?

Same dtype when initialised with same dict-keys.

Steps to reproduce

import km3pipe as kp

t1 = kp.Table({'a': [1, 2], 'b': [3, 4], 'c': [5, 6]})
t2 = kp.Table({'c': [1, 2], 'a': [3, 4], 'b': [5, 6]})

print(t1.dtype)
print(t2.dtype)

print(t1.dtype == t2.dtype)