Add more automatic checks for operators

See merge request !368

Add total_volume property to DomainTuple

See merge request !359

Add 'log10' functions to NIFTy

See merge request !365

docs/source/volume.rst: Explicit HT convention

See merge request !363

Allow for non-blocking plots

See merge request ift/nifty!362

Explicitly state the convention used for the harmonic transform
operators and clearly state the difference between the convention used
in HEALPix and NIFTy.

Pass the `block` keyword on to `matplotlib.pyplot` as to allow for
non-blocking interactive plot windows to be created. This is useful for
creating several plots in fast succession without the python script
blocking the current process.

operator renaming

See merge request ift/nifty!358

Operator spectra

See merge request ift/nifty!333

Although it is working