Lightweight fields

[damonge]

When initializing an NmtField, a bunch of things get stored in it that are only needed if you want to compute the deprojection bias. These include:

• The maps themselves
• The mask alms
• The template maps
• The template alms
  The resulting object sometimes can end up being quite large, which is not always desirable (since we don't always want to compute these things).

This PR will make all of that optional.

TO-DO list:

• Implement in C for curved sky
• Implement in python for curved sky
• Implement in C for flat sky
• Implement in python for flat sky
• Write C-level tests
• Write python-level tests
• Documentation