Add deprecations for deprecated APIs or functions

[chaoming0625]

For example, if users use brainpy.math.csr_matvec, it will give a deprecation warning:

import brainpy.math as bm
import brainpy as bp

csr = bp.conn.FixedProb(0.2, pre=10, post=10).require('csr')
print(bp.math.csr_matvec(1., csr[0], csr[1], bm.random.random(10), shape=(10, 10)))

D:\codes\projects\BrainPy\brainpy\_src\deprecations.py:34: 

DeprecationWarning: brainpy.math.csr_matvec is deprecated. Use brainpy.math.sparse.csrmv instead.
  _deprecate(message)

[ztqakita]

Great addition