Enable setting test size individually for each system

[marian-code]

When one is dealing with multiple systems which have very distinct numbers of frames setting number of tests as only one integer which is same for all systems is rather restraining. For example consider two systems:

1. system with 10000 frames
2. system with 100 frames

Setting number of tests to 10 should be ideal for the second system but is far too low for the first one. There is no good solution to this situation, since increasing test number to say 100 would be appropriate for the first system but wrong for the second.

The change I propose simply allows to set numb_test parameter as a list with separate value for each system or as a string specifying a percentage of the systems frames that should be used for testing.

In the first case, number of tests could be specified in json as numb_test : [1000, 10] or "10%" which would amount to 10% of testing frames for each of the systems respectively in both of the settings.

This requires addition of only a few lies of code and minimal new logic, all changes are commented in the code.

I am pushing this to devel branch, since I don't know your workflow, but if you accept it can be merged directly to master too as it would be nice to see it in conda and docker as soon as possible.

I also made a few minor changes to README which I think serve to better clarify the purpose of json settings.

[njzjz]

The unittest was not passing here:

deepmd-kit/source/tests/test_deepmd_data_sys.py

Lines 79 to 100 in 59d780c

	def test_get_test(self):
	batch_size = 3
	test_size = 2
	ds = DeepmdDataSystem(self.sys_name, batch_size, test_size, 2.0)
	ds.add('test', self.test_ndof, atomic = True, must = True)
	ds.add('null', self.test_ndof, atomic = True, must = False)
	sys_idx = 0
	data = ds.get_test(sys_idx=sys_idx)
	self.assertEqual(list(data['type'][0]), list(np.sort(self.atom_type[sys_idx])))
	self._in_array(np.load('sys_0/set.002/coord.npy'),
	ds.get_sys(sys_idx).idx_map,
	3,
	data['coord'])
	self._in_array(np.load('sys_0/set.002/test.npy'),
	ds.get_sys(sys_idx).idx_map,
	self.test_ndof,
	data['test'])
	self.assertAlmostEqual(np.linalg.norm(np.zeros([self.nframes[sys_idx]+2,
	self.natoms[sys_idx]*self.test_ndof])
	-
	data['null']
	), 0.0)

[marian-code]

I am taking a look at it, just haven't figured out where the problem is yet.

[marian-code]

Everything should be in order now.

[amcadmus]

The new features of the code will be merge to devel branch.

[njzjz]

missing comma