[WIP] Adding remaining trignometric classes

Following classes need to be added
- [x] `tan`
- [ ] `sec`
- [ ] `cosec`
- [ ] `cot` 

I've updated the `tan` class.
In the `__eq__` corresponding to `tan` I've added a check for `sin/cos` form.
Apart from that it is more or less similar to the `sin, cos` counterparts.