Reorganise main mod

[jwallwork23]

To follow #76.

This PR reorganises the main FTorch module to group things more logically:

1. Datatypes
2. Interfaces for core FTorch functionality
3. Interfaces for operator overloading of tensors
4. Procedures for constructing tensors
5. Procedures for interrogating tensors
6. Procedures for deallocating tensors
7. Overloaded operators for tensors
8. Torch model API

This PR also drops unnecessary imports and makes it so that the device_index argument can be passed as a Fortran integer rather than a c_int.

[TomMelt]

Thanks @jwallwork23 , this is a simple improvement and helps with maintainability.

My only concern is relating to the recently changed int32 types which have now been converted to ftorch_int (which defaults to int32 but could in theory change).

[TomMelt]

Thanks @jwallwork23 , LGTM now 🚀

[jatkinson1000]

Yes, I am in agreement that this is a good idea and happy.

Only suggestion would be to perhaps make the section headers even more obvious e.g.

! =============================================
! = --- Derived types and enums                                              =
! =============================================

To stand out when people are browsing the source.

[jwallwork23]

Only suggestion would be to perhaps make the section headers even more obvious e.g.

! =============================================
! = --- Derived types and enums                                              =
! =============================================

To stand out when people are browsing the source.

Addressed in c2e9c1b.

[jatkinson1000]

Go for it.