Fix for the bug #7343

Corrected the bug by adding solutions returned from
each individual equation to the `sols` set. Previous
error was due to replacing the last solution in `sols`
by the new solution rather than appending it to the `sols`.
Fix for the issue #7342

Change the return type of `diop_solve()` to a tuple in the univariable
case. Also added the checks to ensure that only  `Add`, `Mul` or a
`Symbol` is processed in the `classify_diop()` function. Raised an
error in the other cases.
Replace `==` by `is`
Fixed merge conflicts.

Conflicts:
	sympy/solvers/diophantine.py
Use the shortened version of `isinstance`. Replaced
`isinstance(eq, Add) or isinstance(eq, Mull) or ....` by
`isinstance(eq, (Add, Mul, ...))

Replaced the search for `None` using a for loop by using
`in`.
Use iterators in `merge_solution` instead of using counter
variables to keep track of the next element to be taken out
from `params` and `solution`.
remove str from a test file added
Merge branch 'master' into clear-atoms(Symbol)
merging master with newly updated files
modify ode as to eliminate redundant lines
remove _find_first_symbol as no use
changing constructor as to support Integral and Sum forms
Fix unit system example with solve

The documentation has an example on how to use solve with the unitsystem
module, but there was difficulties due to Dummy argument. As a (temporary)
solution the system is solved before replacing with units.
As a side effect I had to allow non-integer powers for dimensions, since they
may appear in intermediate steps.
Add methodto Dimension to test if all powers are integers
Make UnitSystem callable

The __call__ is a wrapper towards the conversion method of a unit into the base
units of the system.
Make DimensionSystem callable, and extend the call of UnitSystem

Both display the given object in terms of the base dimensions/units.
Merge branch 'master' of git://github.com/sympy/sympy into units

Conflicts:
	setup.py
Remove is_positive attribute from Unit

This attribute was given to allow the simplication sqrt(m**2) to m (where m is
meter), but in practice it was not working. Moreover this property leads to
some troubles when using the subs() method (this appeared in commit a5991a3).
reverted back the constructor.py by removing simplification codes
Merge branch 'master' of https://github.com/Mba7eth/sympy into comb_cheat